Time to upgrade - which direction to go?

Hello,

My old faithful Poweredge 400SC is still kicking, but she feels so tired these days. Even though I have her outfitted with Windows 7, 4GB memory and SATA drives, I'm finding myself frustrated at load times and response times on the web. My wife's Latitude smokes my Poweredge, and she's on the wireless connection...

So, I find myself looking at doing a build. I'm not a gamer, so high speed graphics isn't important. My one gaming treat is online chess. I'm a power business user and need to be able to run a handful of biz apps along with multiple browser sessions on dual monitors.

I'm trying to get a handle on which CPU family to look at before I choose a motherboard and ram.

I would like to be able to stay around $300-400 for the build (I have a case and PS ready to rock). Based on my usage, what would give me the best bang for the buck?

Thanks in advance for your help!
Rick