When i went to buy a new graphic card, they said that the newr cards dont support my mobo? why?

my dell dimension 4600 has a agp slot in it,the highest you can go is 512mb video card agp, with a power supply of 450 watts,the processor intel p4 which includes a 512mb agp card is good for older games not for the 64 bit games,but mainly good for music videos,dvd movies, websurfing, youtube, etc.for less than 300 bucks you can upgrade to a mobo and processor combo {intel duo core processor}, and a good graphic card ,check at tiger direct.com,newegg.com,directron.com ,reminder the agp cards are being phase out and being replace with the pci -e ,and pci video cards