Chris, HK's avr 7300 is the current flagship model. This model replaced the avr7200. Both of these models have a torridial transformer, the best type for a power supply. The avr635 replaces last years model the avr630, both are nearly identical. For more info see HarmanKardon.com E.Ramsey
One feature the x35 series has over the x30 series is the A/V delay. For people using front projection systems (especially with DVD players that upconvert and/or the scaling associated with some HD broadcasts) this can be an important feature to be able to sync the audio and video. It's absolutely a requirement for me for the basement system I'm building at the moment.
There is actually a big difference between the AVR630 and the AVR635. First of all, the AVR635 uses the new TI processing chip which is faster and has more processing power than Motorola's top of the line chip. Secondly, the AVR635 has a parametric EQ system called EZset EQ - the AVR630 only has EZset. EZset EQ is a fantastic EQ system that takes 2 sets of measurements of your room acoustics (near field and far field) and adjusts those frequencies that are causing nulls and peaks and distorting the overall sound quality of your room. Harman's EQ is unique in the industry as it actaully adjusts frequencies below 80 MHz, where they are needed most. This means it EQs your sub. Denon's AVR5805 is the only other receiver in the marketplace today that can also EQ a sub.
I've had both the 630 and the 635 and it is no contest, the 635 is my favorite receiver to date (and I have owned a lot of them over the past couple of years - separates as well).