No probs, the only reason I know all this is I'm a Sonyphile myself and have also been looking to change.
Note that although the 5500 series is still being sold in Sony shops, I think the 5710 is newer ( ie 5710 without Freesat, 5810 with Freesat ).
I've done a model comparison between the 5500 and 5710 and technically they seem identical, so maybe just a newer casing design. Might be worth going into your local Sony shop and checking this out - most of the guys in these shops know their stuff, unlike some of the staff you find in Comet/Dixons/etc
Think'll you'll be able to pick up a 5500 cheaper than a 5710, but if you're someone that wants the latest model than the 5710 is the one. Note that 3Fiddy posted a link to the 2010 model the 37EX403 - couldn't see any info on whether 50 or 100Hz, there may well be a whole new range of them.
Gus