Both the 990 and the 7125 will represent sound quality upgrades over the Yamaha - the 7125 because it (like most separate amps) is a better amp than the amp section included in the receiver, and the 990 because of the better analog section, DACs, and build in general (there may also be differences in the DSP, although they shouldn't be as pronounced - everybody has to pass certification by Dolby and DTS, so the variation seen in that process is going to be more subtle). If memory serves, Doug917's processor shootout included that very Yamaha receiver (used as a pre-amp), and the 990 won for him.

That's the easy answer. The question of which is better between 7125 and 7500 is more difficult. The 7500 has more power reserves for dealing with transient signals (brief spikes in the signal that need sudden bursts of power), so it may be more capable of keeping up with very dynamic material (either music or movies) at higher listening levels, but how noticeable that is becomes difficult to quantify (at least for me). On the one hand, Outlaw's amps have a great reputation so either option will put you in a good position (and the 7125 will put you there with an extra $500 in your pocket and less of a backache from setting up your new gear). On the other hand, some people make an argument in favor of buying extra power just to be on the safe side (sort of an insurance against buyer's remorse and against future speaker upgrades).