My current setup for 5boxing on one computer is:
Asus P5QL Pro
Intel dual-core 3.0Ghz
2 sets of dual channel 2x1Gb kit DDR2 800MHz (4gigs together)
ATI Radeon HD 4800 Series (can't really remember which one)
OCZ 2 60Gb SSD
and I can 5box easily with low settings without unnecessary addons.
also this computer can run 9 accounts with decent framerate with some lag spikes (pagefile swapping from SSD)
If you want to do it cheap and without ultra settings
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