Graphics card help

[dell380]

guys i have dell precision 380(3.2ghz 2mb cache,4gb ddr2 ram,nvidia fx quadro 3500 256bit 256mb ddr3,d955x board,375 watt supply)
i want to spend a little money on pc so i can play games should i buy a ati 4670/9500gt (1gb ddr2 thats my buying range)this would help me gaming or i sould buy a dell optiplex 760 with e8400 processor(3ghz,6mb cache,4gb ddr2 ram,nvidia fx quadro 3500 256bit 256mb ddr3)or buying this pc will be good?

 

[manu 11]

lga 775 is dead, just get the 4670

 

[dell380]

it will work on my pci x16 slot?and i have 375 watt psu?
and another if i change a processor to quad core like q9xxx