Your biggest problem will be you PSU. Since you are running integrated graphics, almost anything will be a big step up.
The main things to know about GFX cards are the interface (PCI, AGP, PCI-E), the wattage it needs, and whether or not it uses the memory of your computer (yes, some use both its own memory and your systems, but only low end cards would have this).
128 bit, 64 bit, whatever bit, is the speed at which the card communicates with your motherboard. The higher the number the better, but with you running a 3.56 Celly it won't make a big difference to you.
GDDR2, GDDR3, DDR2 DDR, are all the types of RAM used on the cards. This is completely independent of what you system uses, so it doesn't matter if they are the same. DDR < DDR2 < GDDR2 < GDDR3 etc.
This would work:
Newegg.com - BIOSTAR V8402GL26 GeForce 8400 GS 256MB 64-bit GDDR2 PCI Express x16 HDCP Ready Video Card - Desktop Graphics / Video Cards
This might, but I'm not sure about its power requirements:
Newegg.com - ASUS EAH2400PRO/HTP/256M Radeon HD 2400PRO 256MB 64-bit GDDR2 PCI Express x16 HDCP Ready Video Card - Desktop Graphics / Video Cards
8600GT calls for 300W+.