|"video's start to lag after reaching the 50% CPU cap (which i believe comes from fully using a single core?)"|
You must be using some pretty old codecs. I've never used MPC, but I can recommend VLC. It is multi-threaded, so it should use both CPU cores. But an even better H.264 decoder is CoreAVC. It's so efficient that I can play 720p MKV files on a 1.8GHz PIII. CoreAVC isn't free, but perhaps you know a little more about obtaining software for free than I do...
"Same for games, single process, caps at 50% cpu, starts to lag."
Older games were mostly single-threaded, but I suspect that your video card is holding you back.
"which is AVG free btw"
Microsoft Security Essentials is much better.
Socket 939 | Dual-core Opteron 185 @ 3.2GHz | 4GB | 1.0TB | Win7 x64
GeForce GTX 460 1GB | X-Fi Ti Pro | A8N32-SLI Deluxe | Antec P182