8600GT or 7800GTX !!!!

I will be buying a graphics card within a week.My budget is max 5k for the card.I was thinking about buying the 8600 GT because it is DirectX 10 and comes within my budget.But I heard that this is just an entry-level card and does not play games too well.Meanwhile a friend of mine has offered me his 7800GTX for a good 1000 bucks less.He has two 7800GTX and wants to sell off one.Now please tell me what to do.Will buying the 7800GTX at 4k be a good value 4 money?Or should I stick to 8600GT?I am not a hardcore gamer,but enjoy gaming now and then.I also like to check out new games released like the recent NFS Prostreet,Bioshock and Assassin's Creed.I also want to install Windows Vista Ultimate.I am waiting for your valuable suggestions.