The FTB Forum is now read-only, and is here as an archive.
To participate in our community discussions, please join our Discord! https://ftb.team/discord
This comes in £30 over budget if you would rather have Windows 8 instead of Linux or don't have a Windows key laying around. Without a Windows key, its actually £50 under budget. http://uk.pcpartpicker.com/p/30sZ
You could save enough money to upgrade the video card and/or get a Windows key if you recycle the hard drive from your previous computer.
I've running FTB on a acer aspire 5470G and I have nearly stable 40fps, specs: I5 460M, ATI MOBILITY Radeon HD 5720, 4GB RAM. non overclocked CPU! If I'm travelling around only the fan has to work hard xD
If you are doing a custom build and only play FTB/MineCraft on it I would say look into Linux as an alternative to Windows. It's free so there's no harm in trying.