What to rebuild in a PC?

An update to the 20K will not just want to replace the card and buy a hd 5770 but here's the problem,I have a 330 watt PSU,I think to buy the 500,and here again the problem I have Board p5g-mx 1066 it does not have pci-ex 2 and I'm afraid the map will not rise,what do I do?Or is there an analogue of this card with the same capacity?
Accessories
Core 2 duo e6700(Stok)
3 gb ram(1x1 gb and 1x2gb 400mhz)
At the moment, the map gt 210