I'm not sure if you made a typo when you said the Pc has a Pci slot but you do need a Pci-e slot. Also what style is the case , is it a mini-tower , mid-tower or a slim case. It does matter if it's a slim case because the power supplies that are made for that are specificly for a slim case.
If you have a tower style case then you could get a 500w or 550w or 600w for a relitivly low enough price so as to not make the upgrade too expensive.
CORSAIR Builder Series CX600 V2 600W ATX12V v2.3 80 PLUS Certified Active PFC Power Supply
$69.99 and a $10 rebate that makes the final price $59.99http://www.newegg.com/Product/Product.aspx?Item=N82E16817139028
EVGA 01G-P3-1361-KR GeForce GTX 460 (Fermi) 1GB 192-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
$144.99 and a $25 rebate that makes the final price $119.99.http://www.newegg.com/Product/Product.aspx?Item=N82E16814130749
If you went with these two parts for the upgrade then your final cost would be $180 , so I don't know what your budget is since you didn't list one and I don't know if this is within the budget but you can certianly look for other bargans and lower wattage power supplies.