A 4 year old motherboard has failed. I can reuse the CPU and graphics card in a new MB, but the memory is PC-4200, 533 MHz. All new MBs seems to start at 800 Mhz FSB. Can I use my memory in these boards?
No, you cant. They probably cant work at 800mhz but this aint a big problem, since ddr2 800mhz are extremely cheap right now. Also, tell us what cpu and graphic you have.
^This is incorrect
If you look at the motherboard specs on Newegg they usually just list the fastest memory that the board can identify or has been tested, usually they don't have a problem with slower speeds. Look through the manufacturers sites and the specs are more thorough. List the CPU you have and I'm sure someone here can help find you a board that will work.
Sorry, my mistake.
I read that as your new motherboard
Defiantly AVOID that, and all, nVidia based motherboards.
They have nothing but issues and are usually overpriced for what they offer.
Hell, for just a little more than the price of that motherboard, you could get a New Motherboard, Faster CPU and RAM ($133.97 combined) which would be a LOT faster than your current rig and overclocks like a champ.
For not much more than that, you could get an even faster Athlon II x2 based system.
Do NOT spend $100 on your current system without upgrading.
If anything, get the absolute cheapest s775 motherboard from Newegg and call it a day.
Here is an inexpensive ATX option (although you are just $10 away from a similar Gigybyte motherboard at that point), and Here is a listing of other full ATX s775 motherboards.
FYI, micro ATX motherboards are able to mount in full ATX cases by design so there is no inherent compatibility issues.