Well it sounds like you definately need to go dual core. Thats pretty much the norm on all computer today anyways I guess. Most of the systems your going to be looking at in best buy are going to come with Vista as the operating system. Here's what you want to get a good experience IE not dragging and quick snappy performance.
You want a Core2duo processor for the best CPU. Most of them will be configured with most likely E6400 processors which will do you just fine. You'll want to make sure you have at least 2 gigs of ram. Most are likely to have 667Mhz ram in them try and look for one that has 800mhz ram in them.
3rd you want a dedicated Graphics card do not buy anything with intergrated video. Not sure what the norm is for harddrives as far as prebuilts i'll assume 250gig-400gig drives. If you use it as a media center you'll want at least a 400 gig drive that should give you a easy 125hrs of high quality recorded tv or alot more radio stations recorded.
I'd recommend seeing if they have the 64bit option of Vista Home Premium you'll get a better experience since you'll be recording lots of radio stuff. Much more user friendly the 64bit just future proofs your PC by not locking you in down the road at 3.25gigs.
That should help you out some. If your feeling nervous about possibly building your own check out this thread I wrote on building your own PC. Its really not as hard as people think the only downside is you don't have that 800 number you can just call if something goes wrong you have to troubleshoot it yourself and contact manufactures etc. In the end though I feel its worth it cause like I said you can then tweak it to preform to a much higher end system.
A simple analogy would be you buy a new car with a 5 year warranty. Your not allowed to do anything to the motor etc or it voids the warranty. Custom building your not limited to that and can literally turbo charge your system to dust computers running the same hardware. Only difference would be your system is tuned. But anyways heres that thread.
Hope that helps some feel free to PM me if you have any questions. If you find a system you like you can post a link and i'll let you know if it should meet your needs.