Video upgrade help

Much will depend on what power supply your system has and if you're also going to upgrade it. With the stock power supply you're probably looking at a card such as a Nvidia 8600 GT DDR3 or an ATI 2600 XT. If you add a power supply then almost any PCIeX16 1.0 or 2.0 card would work......as long as it fits in your case. The onboard video will shut off when you put a graphic card in PCIex16 socket. You might have to measure how much room there is at the socket to see what will fit.

Yes, almost any PCIe card can beat to dust an onboard graphics solution. That's talking low-mid and up (GF7K and HD2K series).

Now, the PSU thingy: from what i found, looks like you have a 300W PSU, with doesn't cut it. Get a 400W+ PSU and upgrade to a 9600GT or a 3850 IMO (or anything on that price range). That would give you a HUGE boost.

Also, disable the onboard video once you get the card installed on your PCIe. If you don't do that, u'll be running with 2 video cards and i'm guessing u don't want that.

Esop!

A 300W PSU is sufficient for an 8600GT or 2600XT; neither requires an additional PCI-E power connector. What games do you want to play, and what is your budget?
Vista to XP-Pro is, at worst, a lateral move. If it was Vista Basic, then it's probably an upgrade based on features. If your PC has no more than 1GB of RAM, it is also an upgrade for performance reasons. Apparently, enough driver issues and bugs in Vista have been squished that if you have 4GB or more, then it may have made no notable performance difference.