I've decided that multi-core processors are not all they're cracked up to be.
Particularly when a CPU-intensive game doesn't see the second processor core! Meaning I've got one core almost idling while the game loses up to 20 FPS in parts and along with it any kind of challenge. Those tend to be the hardest parts too, which means I'm effectively playing with a handicap during the game's hardest segments! I mean, what's the point of trying to beat the game in that case? 
And yeah, it's the CPU, not the RAM. The RAM's barely at 50% when I'm playing, while one CPU core is maxed out.
This is a supreme annoyance. Time to upgrade the processor then... but they're expensive... 
Depends on the socket. If you have an P/G965, you MAY be compatible with cheap Wolfdales. If you have P35 you SHOULD be compatible with cheap Wolfdales. If you have P45, you WILL be compatible with cheap Wolfdales. I've got an Abit IP35-E ($80 P35 board going on 2.5 years ago) and bought a brand new Pentium E6300 Wolfdale (2.8GHz; 266*10.5). Compared to my old E2140 @ 3.2GHz, I gained ~25%. There's very little faster in single-threaded and dual-threaded programs than a Wolfdale. A new i3/i5 dual-core may be, but you'd probably have to replace motherboard & RAM.
I played lots of Left 4 Dead 2. I love Scavange mode, doubly-so as Infected. I had a good team the last match and we kicked butt. We were a touch lucky that there were 3 of us at the end, when as a Smoker I grabbed and dragged one of two survivors and made them drop their gas can. I had the most damage for most of the rounds and we won all of the games.