If you want to know your maximum AGP speed you'll need a program like Everest or SiSoft Sandra.
The difference between 4x and 8x is minimal, so don't worry about that. Your computer will most likely support 4x, and I advice a GF660GT, as that card should be able to max out your computer. An X850 is complete overkill for 2.5 year old computers, that money is better spent elsewehere.