Compatibility

BIOS - first things first, if you're planning on upgrading your hardware you should probably install the latest bios. The last official bios at the time of writing is 3.10 I'm assuming ASRock won't be releasing any updates - grab it here:
http://www.asrock.com/mb/download.asp?model=775dual-vsta

Better yet, I installed the unofficial 3.19 BIOS which adds support for more video cards, more CPUs and more RAM:
http://www.pctreiber.net/


Video Card Compatibility:
There's a list of compatible cards on the ASRock website but its quite out of date. Using the 3.19a bios I'm happy running a NVIDIA 9800GTX and I have a friend that's using a GTS 250 without any problems (both PCI-e of course)

For completeness I've also used a NVDIA 6600GT PCI-e, an ATI X800 AGP and an ATI X800 PCI-e. All ran without issue.

I briefly tried a PCI (not PCI-e) FX5500 which worked great on its own but I had no luck getting it to run with the 9800 GTX installed. This was under Windows 7 and I'm guessing its the old drivers for the FX5500 (they're no longer updating drivers for the 5 series).

I purchased an EVGA 9400GT PCI card and I was unable to get the card to be properly recognized at all under Windows 7 64bit (although it should be compatible). It always gave me a PCI-to-PCI bridge driver problem in the device manager. The card works fine in other systems. I don't recommend trying this card on the 775Dual-VSTA. I tried all 4 PCI slots. I suspect the 3.19a bios doesn't properly support it.


CPU:
I recently upgraded to a Core2 Duo E7600(@3.06ghz) from an old E6300 (@1.86) and it works perfectly. Running a very mild overclock to 3.1Ghz - I haven't bothered testing to see how far it'll go.


Memory:
I've installed 4gig of DDR2 memory (I have 2 - 2gig 800mhz sticks). I've been using it like this for a while but apparently bios 3.19a adds better support. My understanding is you'll never be able to access more than 3.3Gig because the bios isn't capable of addressing / accessing it but 3.3 is still better than 2.


Hard drives:
I've ran several up to 640gig, both IDE and SATA and have had no problems.


Sound Card:
I'm running a PCI ASUS Sonar and it works great.


Notes:
I have 550W power supply installed and I'm running dual monitors. Everything else is pretty standard.