As far as how much memory to bump in your PC, if you run Win-XP, don't bother with anything above 2GB, Windows is actually slower with more than 2GB.
When installing RAM of different vendors, this can be risky, as some does not support certain RAS & CAS timing values, that can cause havoc, I don't know how critical this is with newer DDR2/3, but seen many PC not wanting to run DDR400 at 400MHz without blue-screening, and then only runs @ DDR333.
This especially sucks on AMD machines, since part of the AMD strong suit is its on-die memory controller.
Bryan, your PSU was probably, giving trouble, and the extra work-load caused it to really revolt against you, a HDD will not do that, except if it can somehow generate a load of amps, which it doesn't do.
AS for processor choice, currently Intel is kind, the Phenoms are a bit disappointing, only thing keeping them in market is they undercut Intel's equivalent performance cpu's in price, here in SA in any case.
At this point I think AMD's fastest Phenom struggles to put down half on benchmark that Intel can, but then again, that AMD chip is a fifth of the Intel's price. Apples - pears I suppose.
The new opterons seems fast, have seen independent tests, wow there is an AMD in the top five. Not yet available in SA.
I've replaced a load of PSU's in the area where I stay, I've found, funny enough, A-open units last the longest. Our power sucks, and jumps up and down the whole time. The PSU's supplied with cases here, you'll be lucky to get 10 months service from them.