I'm speccing a system up at the moment too. I've looked around at a load and will probably be going for the Onkyo 606 as well.

Other similar priced units don't have as many HDMI inputs [onkyo has 4]...

1. PS3
2. Sky HD
3. DVD/CD player - for her to play DVDs on without fucking up my PS3 and CDs without having to turn the TV on to see what track you're at.
4. Future expansion, projector maybe?

I don't know a lot about all this stuff but the main competitors around £400 seem to be the Denon 1909, the Yamaha AX763 and some of the Sony gear.

I don't know the full specs of the others to fully compare but the 606 does use HMDI 1.3 as said above and also upscales other inputs to 1080i.