This is the slot you want (usually dark brown on top of the three white PCI slots).
If you go with the ATI card, then I would suggest the 9800 Pro version for the 9000 series, or go with the X700 Pro if you can afford $200.
For the 9800 Pro version, you can either pick-up an ATI card, or one made by another manufacturer with the 9800 chipset. The other manufacturers usually use the "Pro" naming also. Some other manufacturers = ASUS, PowerColor, MSI, Gigabyte. I would opt for either an ATI card, or an ASUS card here.
I have not used nVidia, but I do know they make the chipsets only - so you will be looking for a card manufactured with their 5700 or 6600 chipset. Some manufacturers = ASUS, BFG, eVGA, MSI.
Hopefully someone will chime in on the better manufacturers using nVidia chipsets.
Off-hand, I think you are looking at about $90 - $200 depending on which chipset you choose, and whether you find a deal or not.
The ATI 9550, or the EVGA 6200 would both be decent cards.
I would steer clear of the Radeon 9250 for the simple fact that it is becoming obsolescent.
The 64 MB shared RAM for the integrated graphics means that the chipset allows up to 64 MB of RAM to be utilized by the graphics adapter (usually 48 MB RAM are dedicated to the adapter, with the balance available). This means that the RAM comes from your physical RAM sticks, and therefore reduces the system RAM available for the OS and applications, which will force use of the page file (virtual memory) sooner.
Minimum suggested memory on the new video card would be 128 MB, with 256 being the option to take if you can afford it.
There is a thread around somewhere in which people have posted their video cards, drivers, and compatability with WB5.