Hmm lotta good suggestions, heres mine.
You can get a really nice case from atxcases.com at a really nice price. I live by pricewatch, but its true, make sure you stay away from shotty companys, learn what companies produce the best of what products.
We just built a computer for a lady on a very limited budget, built it for $580 total, thats with 17" monitor.
-XP 1700 OC'd to 1.46 (i think) ( it was somtihg around there.)
-Asus A7s333 (i got the board for $52 off pricewatch. Retail Box)
-20gig HD (If your still on dial up, dont need a large HD, your never gonna fill it up)
-Enermax Powersupply (BIG BIG BIG nono is to buy cheap ps, i only buy enermax, they have proven time and again to be the industry leaders is not only quality but price as well, a 350watt enermax you could pick up for like $50 and it would last you threw this computer, and probably the next.)
If you start getting into OverClocking, your definatly gonna want a decent power supply, as you will need it. You can easily get more bandwidth from your ram by puitting some heatsinks on them. Keep everything cool.
Suggestion 2 - Buy a Soyo Dragon kt-400 Ultra Platinum edition mobo, and 1 gig of pc 3200 ram. that should about eat up that budget
you really do have to be carefull of what companys you buy from and who they are made by
Such as - if your gonna buy a radeon card, ONLY by BUILT BY ATI
, if its powered by ati, its not gonna hold strong.
When buying ram, make sure you buy it with a cas rating of 2.0, 2.5 is getting pretty slow these days, and you will regret it soon.
Read up about ECC/parity and non parrity. Do you want error checking, sometimes comanies will sell you unallocated memory
Learn your bus speeds!! Theres no sense in running pc3200 or pc 2700 ram if your bus lines arent fast enough, it will just waste it, and your money.
Wow, sorry i got carried away,
bottom line is, putting a computer together really isnt all that difficult, its just making sure you have the right parts.
Good luck man.