wow... an emachine... eh?
samsung 735df around 100.
256 mb crucial ddr pc2100 around 72.00
40gb harddrive around 80.
16x burner around 40.
so far 292
so i still have about 300 more to spend on mobo+cpu+case+psu.
if i want speed... i'd go with P4 1.6a northwood for 150
and match that up with asus p4333 for 112
that leaves me about 38... for case and psu.
i'd actually spend all that 38 on psu and go bare naked.

300w enhance ENP-0730 for 39.
oh wait.. i forgot about the v-card..
so spend about 50 on gts-v card.
all in all... you've got yourself at least 2-3 times the computer than what you have with emachines.