Well, it's going to be tough, and it wont be top-of-the-line, but I think this may help you out.
Here is what I put together.
I can vouch for some of those components.
The motherboard is in my parents PC, as well as the HD, and the cooler. The RAM is also in there, but in the PC2700-flavor.
My experiences have been good.
Gaming; I had a Halo party thing at my house, one of my friends used my parents computer. It ran it. It wasn't full eye candy, granted, but it ran it. And, it's only a 'temporary' fix. The motherboard has a 4x/8x AGP slot, so when you get a few more $ you can grab a nicer, actual, card. However, for onboard, it's excelent. Did better than my brothers FX5200 on a P4 2.8C @ 3.45GHz at Halo.
On the note of the motherboard, absolutely wonderful board. BIOS is excelent, stability is awesome (have a 2400+ XP-M running nice and cool at 2.05GHz, no voltage adjustment or anything), and on the note of overclocking, one of the best mATX you can find for it.
The HD is pretty quick, great price. Recommended.
The RAM we have in there, same stuff practically, only rated at 2700 from the factory, is running 340MHz @ 2-2-3-5. Stable as a rock, great stuff.
You could get a different case, I jsut threw whatever was cheap and had what appeared to be a decent PSU. You can get a mATX case if you'd prefer, make a smaller computer. That's upto you. I'd splurg a little and get a snazzy case, though.
The CPU. I don't personally have any experience with a Sempron chip, but for a budget PC, they appear to be decent. The 2600+ should provide fairly decent performance.
Didn't get a fancy DVDRW or anything, that's all stuff that can be added as needed, as funds permit. This is a decent base system for what you wanted.
Shipping will bring it over $350, but should leave nearly enough for XP Pro to not hurt you too much. If you can get an acedemic copy of XP Pro (if you're a teacher, or student currently), that'll save you some money. My brother can get a license of XP Pro for like $7 at his college. If not, just look around for an OEM XP Pro at a decent price. Should be right around $500 overall, maybe a little more.
Click 
