There is a guy on ebay that is willing to sell me a xfx nvidia 7800 gtx graphics card for about 140$ total, know my question is, should I just go with a 7600 GT that i can order through an official retailer for 100$ or is it worth the extra 40$ to get the XFX 7800 GTX instead?

Go with a new card, A 7800 for 140.00 when you can get a 7900GS for around 159.00.
http://www.newegg.com/Product/Prod [...] 6814150202

dito for me too! go with the 7900gs if your around that 150 price range.

I wouldn't buy a used 256MB 7800GTX for that price.

I'd go with a X1900GT for $130 no rebates:
http://www.chiefvalue.com/product- [...] 14-102-022

or if it must be NV, $140 after $15 rebate for a 7900GS.
http://www.chiefvalue.com/product- [...] 14-130-056

this is a stupid question but the ATI card will work ok if i have an nforce motherboard with an integrated nvidia card?

Yes. Depending on what board you got when you plug a video card in the pci ex slot or agp slot the bios see it and turns off the onboard video and uses the video card. and ati cards work on a nvidia board.