I am actually going to DISAGREE with the suggestion of an FX 5200, and suggest a different low range nVidia card
I am going to suggest the GeForce4 MX 440, NOT THE SE VERSION, and if you find a version that supports AGP 8x, get it (so if you upgrade the motherboard, and it's AGP 8x, it can use AGP 8x (instead of getting an AGP 4x only card, which means if you get an AGP 8x capable board, your stuck at 4x...)
the reason I'm going to suggest the GeForce4 MX 440 is this:
I used to own both (I still own the GF4)
playing most 3D intensive games, the GeForce4 MX card was considerably faster
the reason is simple:
the GeForce FX 5200 is a fully DirectX 9.0 capable card, with all the hardware support features required to make it a DirectX 9 part
games will default DirectX 9 level shaders, graphics and other features on it due to it's capabilities, and it will usually run slower than a Pinto in gaming due to that...and it doesn't do so well with those features disabled (dropping it into DirectX 8.1)
so here is where the GeForce4 MX 440 comes in
it is a fully DirectX 8.1 compliant part, with no abilities to run DirectX 9 shaders (BUT it can play DirectX 9 games (it just doesn't have all the eye candy DX9 offers, only what DX8.1 can give you)
so now that we have leveld them off
both are in DirectX 8.1, and both are nVidia parts of simmilar design (4 pipes, 64-128MB of RAM, 64 or 128-bit memory interface, basically their the same)
BUT
there is one key difference
the GeForce4 MX 440 is clocked higher, so it can achive higher fill rates in games, meaning more FPS, meaning better performance
it's 275/333 for the GeForce4 MX 440
compared to 250/400 (or 250/333 for the 256MB version) for the 5200
when you overclock the 5200 to 300/425 (I had the 256MB version @ 305/425 up from 250/333) the performance difference switches around, and the 5200 wins
but imo if you can buy the GeForce4 and not have to OC it, you should
I am going to suggest it on the basis that it will out-perform in most cases
now i know Tom's Hardware states the 5200 is faster, and that is the general consensus
but in my experience the GeForce4 MX is faster (AthlonXP 2600+, GeForce FX 5200 256MB, 512MB PC2100 DDR vs. Pentium 4 2000/Willamette, GeForce4 MX 440, 512MB PC133 SDRAM, and the GF4 equipped system wins in Halo by around 5 FPS, and in most other games it did admirably)
but 5FPS, while neglible
means playable and not playable on these amazingly low end cards, as most of them play between 15 and 35 FPS in most games
so my suggestion in the $35-$45 range is the GeForce4 MX 440 8x
BUT
that is not my final suggestion
my ultimate suggestion would have to be the XGI Volari V8 256MB (it's $100) and it will perform as good, if not better than, a GeForce FX 5700LE
the only issue you MIGHT run into with the Volari is a limited game library (has no issues with current games, i mean if a game is put out next month, it may take them a week to get the game added (not likely, as their doing great as far as game support, this isssue is probably more common in reverse, older games not working with it's drivers...unless they can run 2D or software accelerated (in which case your CPU better be up to it)
also, would you stop suggesting a 6600GT?
considering he is considering replacing a GeForce2 MX in a system with AGP 4x, i'm guessing a 6600GT is probably as fast as his CPU (or close...)
and aside from that
it's just going to be un-needed power
if he is considering a value level card, for a basic performance increase, a 6600GT is like buying a 5950 Ultra...it's just a very fast, very gaming oriented card
i'd suggest a GeForce 6200 AGP or GeForce 6600nU AGP
or the Volari V8 256MB
or the GeForce4 MX 440
or from ATi the Radeon 9600Pro (which should be faster than the Volari)
i can't really think of anything else to suggest to you...the 6600nU, 9600Pro and Volari V8 concern me on their power requirements for your system, as they all want 300W or more
the GeForce4 MX 440 (personal experience talking here) will work on about any PSU (smallest i've run it on is a 200W along with a Willamette P4, 2 optical drives, and a hard drive)
and the GeForce 6200 should be pretty low on power needs
the 5200 should be about the same (if you decide to buy it)
the Volari is probably going to want a good sized PSU, same with the 6600 and 9600Pro