I usually try to go with at least one console and PC. There's generally some different types of games on each that you can't get much of on the other.
Investing in one of the next gen consoles can be a really good in terms of gaming/$$$ over the years...definitely a good idea if you like the games.
It looks like you have a pretty good PC that should work well for a pretty long time except for maybe the video card. Hopefully you have a PCIe slot and have a good upgrade path there. (If not, be ready to pounce on a 6800 GT AGP while you still can.) I don't think you'd need to bother with anything else upgrade wise for a pretty long time to have fun playing PC games. It's good to realize that PC games are still tons of fun even if you can run everything at the highest settings. I usually spend some time up front on a new game trying to figure out what the best settings are for my machine and then sit back, relax, and just enjoy the game.