Hi James
Given that you have 5500s across the front, I reckon the biggest sonic gain for music would be a G processor with room correction. You could find a G68 for ~�2k and selling your 568.2mm would get you to an overall outlay of �1k, ie on budget.
That doesn't get you rears of course, which will add to your movie and games enjoyment. �1200 will get you a set of MkI 5000s in 96/24 trim. Certainly a pair of DSP33s with cash left over (~�800)
Re the F80, I'd go ahead and just buy it. I've had hifi for decades, and the first piece of kit which has sparked my wife's interest in listening to music is the F80. Goodness, she's even bought a few new CDs of her own! She'd happily dump the rest of my system and just live with the F80. Ex dem ones come up for �1000-�1200. And just as an interesting aside, not only has it variously proved itself as a superb one piece system for music and DVDs (with an older 42" SD plasma), it amply filled a sizeable church hall two nights ago with backing tracks whilst my son sang a few songs!
Cheers
George