Can I put PC2 4200 memory (533 Mhz) into a motherboard with 800 Mhz FS

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.

The dead motherboard is an Asus P5GD2 Premium.

The CPU is Intel P4 540 at 3.2 Ghz.

The graphics adapter is ABIT RX600 PRO-128 PCIE with 256 MB memory.

Yes, your 'new' ASUS P5GD2 Premium is based on the Intel 915P chipset and does support PC4200 AKA DDR2 533Mhz RAM.

Hi,

if you buy a motherboard that support you cpu, the memory should be ok too.

However, if you change to a faster cpu, you may need to buy faster memory that goes with the new cpu's fsb.

Brad

Old old board is a P5GD2. I was thinking of buying something a little more current. The XFX nForce 750i at TigerDirect (http://www.tigerdirect.com/applications/SearchTools/ite...) looks pretty good for the money. Has anyone tried one of these?

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.

This is great advice, and I would do it - except that this is a micro ATX format, and I need full size ATX. Do you have any more suggestions?

I'm going to start a thread on my P5GD2. It might be salvagable - I'm getting some terrific help from Tom's Hardware.

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.