Welcome to CF! BenMinor
When looking at ATI.AMD I don't even come up with a 4650 but see a 4550 listed instead for different brands of cards at their support site. You can't expect full performance out of either of the new HD type cards intended for newer boards then the old Socket A/462 model you have there.
The personal assessment here would be to save the money and put it towards a much newer system you will eventually upgrade into rather then tossing too much on an outdated set of hardwares.
PCIe or PCI Express is where you will see actual performance gains over the AGP bus to start with. If you simply looking at replacing the present card and not looking for any great performance gains shop for one of the older models for a lower price until you do move into a new system.
I just a Radeon HD 3650 Sapphire model work a look for quite a bit less at http://www.newegg.com/Product/Produc...82E16814102814
For the old build there you wouldn't need any more then that. In fact a friend still running an old Asus A7V model board with AGP there just had to replace one card with a 3650 for the old XP build to keep that going.