Logitech Z 5500 THX system is actually 6.1, but THX approved, plenty of power and sounds great. Also, using the mapping inside Vegas allows using all 6 channels very easily.
You need is a sound device with 8 outputs. The M-Audio Firewire-410 can do 7.1. Obviously you also need 7 speakers and an LFE. I've got the M-Audio Studiophile LX-4's with the 5.1 extender. You can probably just add another pair of powered stereo speakers like the DX-4's for 7.1.
On the consumer side you could get an M-Audio Revolution 7.1 and a pair of Cambridge Soundworks 7.1 speakers.