I Love my Husband so....

First things first, you need to make sure that there is even some kind of graphics card slot in the computer before you go thinking about getting a new one.

So, you can either try emailing HP/Compaq to find out if it has an AGP or PCI-E (PCI Express) slot, or you can just open it up. Maybe get a can of compressed air and blow out any major dust deposits while you're at it. AGP slots are usually brown in color if memory serves, and PCI-E are a kind of light blue in a lot of cases. But just check with Wikipedia or Google's image search to make sure you can visually identify them.

Assuming your system has at least an AGP slot, you can start looking at video cards. Just keep in mind that if you have only an AGP slot you can only use AGP cards, and same thing for PCI-E.

The next thing you need to know about video cards is to disregard the amount of RAM advertised as coming with it. That's largely irrelevant to the performance of the card. What you want to look at are the GPU and memory clock speeds (generally speaking, bigger numbers are better) and also the memory bus size (measured in bits, and again, bigger is generally better). Another important factor is the TYPE of RAM on the card. A video card with 256MB of GDDR5 RAM would be a much better performer, all other things equal, compared to a card with 1GB of GDDR2 RAM. Most of the cards with 1GB or RAM or more will have a pitiful 64-bit memory bus and only GDDR2 RAM.

The type of RAM GDDR or DDR (same thing) determines the speed at which data can be moved in/out of RAM on the card. Higher (G)DDR numbers generally means the memory handles data faster. The memory bus is how much data can written to/read from the RAM at any given time. So, if you think of it like a highway... The type of RAM is like the speed limit, and the memory bus is the number of lanes. Even if the speed limit is say 120kmph, if it's only a two lane road, you can only move two cars through at a time. So imagine if you have a 3, 4, or 5 lanes moving at 120kmph. More cars (data) can pass through a given point at a time.

One final point about graphics cards, which can be a bit of a "gotcha" for people, is that most performance cards require an auxiliary power feed. This might mean you'd need to replace the power supply on your computer if you don't have any free power connectors, or don't have the proper connector on your power supply. It might also be that your power supply isn't able to supply enough power for the video card and everything else in the computer. So be sure to check what kind of power supply the card requires, whether or not it requires some kind of secondary power feed, and what type if any. Match that up against your power supply. You can find out it's "size" by reading the label on the outside (of the power supply, inside the computer case). The key bit of information is the "Output" section, and it will have some number in watts. Probably 250W on a system like yours.

Moving along...

On the audio end of things, you might want to consider an actual surround sound receiver or home theater system. Now I know these things can be expensive, but a lot of makers sell refurb units at a heck of a discount. Onkyo is a prime example with their Club Onkyo site, and others will sell refurb units on ebay stores. In a lot of cases, these are just discontinued models that they're looking to get rid of, so they label them as refurb, sell them at a pretty significant discount, and everyone wins. You get a great deal on something, they clear out inventory to make room for newer units with higher profit margins, etc.

A home theater system can be used for more than just the computer though, and could be used with your television for example. So that helps justify the cost a little more. But odds are your husband will absolutely LOVE the ability to have that full surround sound experience to kind of help enhance the illusion of the game.

But if a home theater system is outside the budget, even at refurb outlet prices, then just make sure you get something that includes a subwoofer. Having a sub to take the load off the main speakers for all those low booming sounds has a real profound effect on sound quality. You can get simple 2.1 (stereo speakers + sub) systems for very little.

I don't really know the answer to the joystick issue. I think most people prefer the keyboard and mouse combo, but I would assume you could make a joystick work if you wanted.

You might also want to just consider a console. Instead of sinking a lot of money into what is effectively (sorry) a crap system might not be all that cost effective ultimately. For probably around $300CN you could get an Xbox 360 Pro, and then games are probably around $60CN for a new release, and possibly quite a bit less if you buy used and/or older titles. The great thing about consoles, is that they are great bargains. You buy the one unit, and every game with that console's logo on it is guaranteed to work on that console. So you can spend more money on games, not the hardware needed to run the game. I think the Call of Duty games have been released for both the Xbox 360 and PlayStation 3. You have about 3-4 years worth of games to select from already on both consoles, plus any future games coming out in the next 2-3 years before their successors are expected. So it can be quite the bargain when you spread the initial cost of the system over 2-4 years. Something to consider along side your initial plans.

That's probably more than enough for you to chew on for a time, and others will likely chime in with other thoughts. I'll be happy to answer any questions you might have about anything I said.