You can download this from Cnet's download section, the program you want to download is PC Wizard 2008, which you can find here http://www.download.com/PC-Wizard-2008/3000-2094_4-10793737.html?tag=mncol&cdlPid=10868055 simply install it and when it first runs it tells you immediately what is your mainboard/motherboard (model number so you can do a search) what chipset it has, your cpu speed, how much ram you have and what type,and your video card type as well, etc. Now that your armed with this information you have to decide is it worth sinking money into an old computer or buying a newer motherboard, cpu, and ram? You can reuse the current hard drive you have though before shutting off the computer uninstall the graphic's driver to avoid a reboot conflict (Xp is tough, after you change out the motherboard, enter the bios, set boot order to CD/Hard drive or whatever, and then have the driver's disk available for the new motherboard and after about 3 reboots you're up and running). Once you identify your board post here and we'll all suggest what memory you should buy or even if you should.
Right now I'm typing on an Intel Atom z530 dual core @ 1600MHz with 2gb of ram which can be put into any tower with a micro-atx form factor (this board has four screws, quick and easy install) which pretty much is any budget tower most people would have bought. I bought the motherboard/cpu combo for $90 and the 2 gb single stick of ram for $25,and reusing the hard drive, Dvd burner, powersupply, and case (I intend to add more hard drives and make this a media file server and will downgrade the powersupply accordingly since it's overkill). If you look around you can do better if you want to do more work (seat a cpu and heatsink/fan combo) and get a better rig (Intel e1200/e1400/e2180 and a cheap board or Amd 4400+ Brisbane/BE-2400 Brisbane 2.3GHz, both brands can be had with a budget board for under a $100 if you shop around, you should be able to reuse everything but ram and possibly video card) for a main duty computer. I wouldn't recommend any single cores though, multicores are the future. Lucky for you most of the budget boards will also have floppy drive (worthless), one ide connector (for CD rom and hard drive), and usually a pair of sata connectors. Another thing you like is that the newer boards support usb booting, with pendrives you can experiment with various versions of linux as well.
UDMA, Ultra DMA, or Ultra Direct Memory Access, it supports direct memory access with ide controller to enable faster data transfers, "Ultra" is a marketting slang for IDE 100/133 speeds.
Sata, Serial ata, a faster interface for data transfers for hard drives, I like the fact it allows better airflow though most cables are way too long. Sata drives and now faster, larger, and cheaper than their IDE counterparts at given comparison points, you'll get more bang for the buck. This is not backwards compatible with most of the older single core models (early p4s and athlons) motherboards usually. Sata 2 is marketting hype, I've yet to see a sata drive that can consistantly deliver on the limits of sata 1, in fact lot of the Western Digital Raptors were the benchmarks and they're sata 1s.
NCQ native command queuing, from wikpedia "Native Command Queuing (NCQ) is a technology designed to increase performance of SATA hard disks under certain situations by allowing the individual hard disk to internally optimize the order in which received read and write commands are executed. This can reduce the amount of unnecessary drive head movement, resulting in increased performance (and slightly decreased wear of the drive) for workloads where multiple simultaneous read/write requests are outstanding, most often occurring in server-type applications. However, the current (as of 2004) technology actually slows down HD access in certain applications, like games and sequential reads & writes, because of the added latency induced by NCQ logic" so it's application dependent.
Dan if you don't mind, give us your intended main intention(s) with this computer, an approx budget, and we'll all pull our hair and argue about the best way to help you spend your money. If you can follow instructions, somewhat capable, and have decent motor skills (main finesse skill is plugging in the front panel wires into the motherboard) you'll do fine if you're open to the option of upgrading.