you Don't have seven megabytes of Ram... Please post what you will be using the card for? light/heavy gaming etc, and also please post the resolution that you will be gaming at? Your other system specs will also help us try and make an informed decision about a recomended upgrade.
With a 300W power supply and a free PCI express*16, you could get a Geforce GT 240 or a Geforce GT 440. These are weak cards for graphics, but a LOT better than your on board graphics. These cards should play fine if you use low resolutions, and medium detail.
Any more powerful cards from Nvidia will require a power supply upgrade. I think ATI has cards with performance inside the gap Nvidia has between the GT 440 and the GTS 450. One of those might be a good option, but I do not know ATI cards.
Also, keep in mind your PCIe *16 is not 2.0 and all new cards today are PCIe 2.0.
This is not a big problem as the cards will run, but at lower bandwidth. That means the cards cant produce graphics as fast as if they were pluged into a Pcie 2.0 slot.