I recently tried to upgrade my oldest PC, which happens to have an ASUS A7S333 MB, and I'm running into a similar issue.
I have 2 Matched 1GB crucial DDR PC2700 sticks, and I cannot get the PC to do much more then load into BIOS when I have them both in. They show up in BIOS but even it acts clunky and slow. Also if I run Memtest86 with both sticks in, it barely loads and then it crawls.
Normally with 1 stick of RAM I see values in Memtest86 such as
128 MB Level 1 Cache 8,000 Mb/s
256 MB Level 2 Cache 2,500 Mb/s
1024 Memory 259Mb/s
(these are approximations)
If I can get it running with 2 sticks of Memory it looks like this
128 MB Level 1 Cache 8 Mb/s
256 MB Level 2 Cache 8 Mb/s
2048 Memory (no speed even listed)
At which point the test crawls so slowly that its still 15% into test #3 5 hours later
Now I have tested each stick thoroughly and each stick works individually in any of the 3 slots alone with no problems. No combination of 2 slots however will seem to work properly.
My next plan is to try updating the BIOS, I believe I'm at version 1006, but I got held up with long days at work and not having a way to create a bootable DOS disk/CD (I just need to install the software and/or find the blanks). I plan to install BIOS 1017 I believe when I can.
If anyone else has any other thoughts, I'd be interested. I was going to try PC 2100 but I don't have any laying around and don't want to blow another $70 on this PC really, when I'd have no other use for the RAM if it doesn't fix the issue.