Dual boxing help

[mykro9]

To start, I looked through some general forum topics, and tried to do a search fo dual boxing, but typing 'dual box' brings up threads with the word 'dual' or 'box' but not necessarily both. So if I could get help on how to find the thread I need first it would help.
Second, I have an older PC, like Core Duo old, running XP. I have 3 accounts and routinely play at least 2 at the same time, but I am noticing that I am having FPS problems, with weird lag at times.

What I want is to spend the least amount possible to get a new system to play 2 accounts at the same time, windowed where I alt+tab between the 2. What I would like is some recommendations on what is the most important parts I need to look for. I have built PC's before, though obviously it has been a while, since this was the last box I put together.
I have a 128gb SSD I bought over Xmas that I want to use, and I will need Win 8, ram, CPU, GPU, mobo, PSU, and case. I have bought Shuttle PC's before, and would most likely go this route again, so that takes care of the mobo, case and PSU. I am not looking for ultra anything, I just want to dual box the accounts and not have lag issues.

If there is something simple I can do with what I have now to fix my problem cheap or free first, that would be best.

To mods, if this needs to be moved to a better section to get help, please help with that, too.

[lockblock]

When I dual boxed in the past I had stuttering issues because I was running out of video memory by running 2 clients at high preset @ 1080p.
To solve this I simply turned down the resolution of the secondary client so that it wasn't maxing my gtx 570 out,
The more clients you have open the more video ram you need.

You can see if that is affecting you by running a program such as evga precision x, msi afterburner, gpu-z or other gpu software and check vram usage before running 1 copy of wow and then again while 1 copy is running.

Almost forgot but you should also set max background fps for both clients to something low.