Well, I have only 1 PC and it's specs are:

Pentium 4 3.0 Ghz HT(hyperthreading) Processor
2x 512 MB RAM sticks (1 GB system RAM)
on-board 224 MB SHARED vid card (crap, i know)
400W Power supply
SATA II 160 GB hard drive
19" widescreen monitor

Now, say I have $100 USD to upgrade it. What would be the best part (or parts) to upgrade for multiboxing / video editing / ect...? Basically, what (for $100) would benefit my PC overall? And where should I get it (like tigerdirect)?