Obviously, I'm new to this sort of thing. Thanks to everyone for your input!
As it happens, the mobo seems to be the problem. I've got two identical sticks (they came as a dual pack) of 512, but no matter what combination I use them in (dual or single channel), I get memory errors. I've been getting these results with memtest86+ since my last post. Either stick on its own tests fine, but together I immediately get many failures on most of the tests. Some people on the memtest86+ forum over at x86-secret have been saying that timings (and some mobos' inability to detect the correct ones) may have something to do with the failures... but since Corsair doesn't seem to publish timings for my ValueSelect's, I've had to guess (all I know is that CAS==2.5) which is probably not a good idea. In any case, I didnt damage the memory with this unsavory approach.
After having success with testing only one at a time and verifying that two will not function, I decided to tackle the original problem of game-playing with only one stick. I'm happy to say that we are finally getting normal behavior.
My next and probably final question is this: would it be incorrect to assume the problem is the mobo? As the sticks seem to be fine independently and they are supposedly identical, wouldnt the board now be suspect?
I'll understand if nobody replies to this. In any case, thanks again!