okay, i have an emachines d6417, and although i didn't upgrade the psu, i am just waiting for mine to go.
I love (!) my x800gto2, as far as a kick a$$ cheap pci e graphics card:
it's fun to mod and all that (although I hear the new ones don't need to be unlocked, which is nice) and it overclocks like a charm (although i hear the newer ones don't overclock as well, but they will still oc pretty nicely)
I get 6600+ (barely
) in 3dmark05, and that's nasty by my standards.
if you want sm3.0, you could go nvidia 6800 series (although 7800gt is coming down in price, but that i'm pretty sure would blow a 300w psu like *snaps*) or ati with the x1600 (which also might blow the psu...). right now, i really like the x1600's, although not near the performance of a 7800 or 7900 gt
also, in these itty-bitty cases, some of the pcb's might be too long. check that out before you buy anything. my x800gto2 blocks the bottom release latches of 2 of my ram slots (msi-7093 mobo), and i'm pretty all the ones i said above (cept 6800 i think) are longer. so check that out with your case and mobo.
good luck!!!!