I am also thinking of putting together a processing desktop, but i am cosnidering the AMD AM3 Phenom II X6 (Six-Core 3.3GHz/3.6GHzTurboCore) for the CPU. Cheaper than an I7. Most people thouh seem to be reccomending Intel for Video editing and Image processing. The i5 2500K seems to be the best processor option at the momement, from a price perforamnce perspective. Even though tthe AMD is 6core the I5 quad seems to take it on with no problem. So I would for most people reccomedn the I5 2500K.
A good place to look at is the whirlpool PC suggestion Wiki
http://whirlpool.net.au/wiki/rmp_sg_whirlpoolpcs
There are 3 configurations given for Video/Photo editing here. Note all but the last one are based on the I5.
http://whirlpool.net.au/wiki/rmp_sg_...asking_configs
Note the prices shown are based on the cheapest part prices found by searching staticice.com.au. But i suggest email a few shops with the config and ask for the best price. Noting the prices given here as the best you have foind. Most shops will price match.