Switching to the two molex connector cable didn't help. Will the card really only cost 500? Would buying a sli motherboard and a 7800gt and using both be better than a single dx10 card?
motherboard board and psu dont play a role in 3dmark scores i dont think. if you do get a 7900GT you will see a strong performance boost, the 7900GT is equivalen to the 7800GTX, which urs is not, so you see what an upgrade that would be. yes ur mobo isnt the best for ocing especially if its one of the older Asus boards. you might find changing it to an MSI K8n neo4-f a benefit if ur planning to OC, which you SHUOLD, its the ezst thing.
You should be okay if you start saving now. And you could always sell the 7800GT for that little bit extra! I dont expect the "GT" version to cost much more than the current generation GT cards, propably in the region of 350 - 400, which should be quite manageable financially methinks!