My home-built is getting old but want to get some more years if possible...MSI 7

Hi you smart guys (gals?) out there... Help an old fart-out here please...

My home-built is getting old but want to get some more years if possible...
AMD Athlon 64 (ordered an Opteron 175 coming-in to replace it)
MS-7125 K8 Neo 4Moboard
3GB RAM
WIN XP still 
My Video card, Nvidea GeForce7800GT has been awesome since I installed it in year 2006, but wonder what I can replace it with? I mean, is there a newer once that would make a noticeable difference? Or would upgrading it be a waste of money?
Can't find web site that says what is the best card for my set-up, So I turn to you for your wise advice and opinions.... I'm really not that old, but I was born before 1960 if that tells ya anything!!!
Sure appreciate any help. Thanks in advance, and God Bless.