How much improvement on new computer?

Hi guys, I'm currently 3 boxing shamans on my (incredibly shite) laptop, and I am planning to get a new desktop soon I am just wondering how much of an improvement I should see fps wise.

Current stats:
Intel Core 2 Duo Processor T5750 (2Ghz, 667 Mhz FSB, 2MB L2 cache)
Up to 1536 MB ATI Mobility Radeon HD 3470 Hypermemory
4GB Ram
250GB HDD

New stats:
Intel Core 2 Duo E8400 (3Ghz, 6MB cache)
ATI HD5750 1GB or nVidia GTS 250 1GB (whichever is better in your opinions)
8GB Ram
1Terabite HDD

As you can probably tell I'm only young and strapped for cash so not much can be done about that.

EDIT: Forgot to mention I currently pull an average of 30-40 fps while 3 boxing.

The newer one will be faster as it is a much newer GPU. I presume you're running relatively low resolution and have your MAXFPSBK set to like 15? With the new graphics card being in a desktop and not a laptop means you're going from UMA graphics to discrete. I would expect you'll see 60+ FPS on all 3 with the settings turned up to high and set to higher resolutions. The jump from UMA (shit) grahpics to discrete is like going from old PCI VGA to PCI-E; it's so much faster as to be in a whole new league.

Yeah graphics are medium on main and low on clones, and maxfpsbk is low aswell.

Well it's good to hear about that upgrade :) I will just have to adjust because I'm so used to playing with laptop on my lap, ain't played on a desktop in years.

The desktop will be worlds away from the laptop. I have my relatively new (close to a year old) with a dedicated video card which I 5 box on from time to time and it is like treacle compared to my desktop which is not that much faster power wise. The big weak link is the video card. When I look at my laptop performance, memory is at about 90% and CPU is about 60% but I still only get max of 20fps dropping down to 10fps on some spots. Desktop is a steady 60fps with the same settings.
Something that also hurts my laptop is that it's screen is 1900x 1620 (I think) LED display and so it has to run the main at that res (cannot find a way to drop the regular res to lower in Windows 7)
You will be really happy with the desktop except for the couch comfort factor :)