mmm, nothing like a cheese burger (cooked by yours truly) @ 1 a.m.
anyways, on to your question, even though you probably know what i'm going to suggest.
I'd say find a case with a power supply (usually comes with a 300-350w) which would be fine average gaming and use.
You'd want a big hard drive because that just means more files you can save! I'd say a 40-80 gb would be fine (unless you're a big time movie/music fan)
You'd want atleast a Nvidia Geforge FX 5500 or Radeon 9200 or better, just so your computer preforms tasks faster as well as being able to handle the older games (nothing VERY recent)
I'd say if your on a budget go with the Sempron 2200+ with a nice socket A mother board that supports AGP 8x and PC2700 or better RAM. I've never had a problem with my AMD Sempron 2200+, its not the fastest but its great for a budget PC.
As for ram, you can either get 2x 512 sticks of PC3200 or 1x stick, either way you don't wanna be stuck with 256mb, especially if you're using the memory hungry WindowsXP.
If you're into watching DVDs on your computer then go ahead and purchase a CD/DVD-ROM drive, they go for about ~$30 USD, and you'd probably want a floppy drive. I know they're out dated but they are very useful imho.
Sorry that no one else is posting but its pretty early over here. hope my post helps the least little bit.