I'm sure everyone will have their opinions and facts of what you should buy... so I'll share my experience and opinions:

For the past year and a half, I've been running 3 characters on a dual-core AMD 4200+ w/ 4GB of RAM. Well, started w/ 2 chars, 2GB of RAM, added another toon, added 2 GB of RAM. Recently, I've been 5x and have been experiencing some fps latency. At first, I thought it was a videocard performance problem. I went out and purchased two 9800GTXs and discovered that you can only use a single monitor w/ SLi. No big deal because I play other games and the SLi is great for those game. It was still choppy at times...and REALLY bad in Shat... in fact, it was almost worse.

Here's why: Adding two videocards ate away at 1GB of my physical address space. Windows XP will let you have 4GB of physical address space. Your memory is part of this physical address space. Basically, I was now down to 2.5GB of RAM because of this limitation. I upgraded to Windows Vista 64-bit.

After upgrading to 64-bit Vista, I noticed some improvements, but Shat was still pretty bad and so were the BG's when a lot of people were around...almost unplayable. So far, I've increased my RAM, purchased better videocards and upgraded to a 64-bit operating system. I ran Vista's resource monitor while playing the game and discovered that my CPU, both cores, were pinned at 100%.

This last weekend, I went out and purchased a Quad Core 9950 AMD processor + MB and 8GB of RAM. Now, I get about 50-60fps on all 5 screens. It was an AMAZING improvement. I would highly recommend, at a minimum, that you get a quad core CPU and at least 4GB of ram if you want to run 4 toons at the same time. If your videocard isn't the greatest, you can always /console maxfps(bk) [some low number].

I don't think you're going to find a popular brand name computer company that can offer a "multi-boxing solution" for $500-$600. What computer were you looking to get? Let me know so I can at least find a base-line of what you're interested in and make some recommendations off of that.