in the specs it's listed as a 90x5 with preouts for the rear channel, am I correct to assume the amp can drive a 5.1 setup but needs an amp to drive ES 7.1? or am I just confused?
You only need an extra amp if you want to power the rear channel for EX material. Without it, you will still have full 5.1 Dolby Digital and DTS sound.
I have the 36TX, which is much the same as your unit except for the additional power and the THX Ultra. Yes, the rear surrounds do make a difference, even with regular 5.1 audio if you have the THX Cinema setting.
For amplification, I would recommend the Pioneer Elite A-35R which is designed specifically for the TX series. It can be had for around $180, and of course you will have to get the speakers.