The new 105 components are excellent. Keep in mind that Shimano consistantly lets the technology trickle down, so the new 105 stuff is pretty close, in performance, to six year old DuraAce. The most expensive bits are the STI shifters and you can get them (9spd) right now for $149 from Nashbar and for $131 (!!!) from Performance. The Performance price is just for the next two days. If you can get discount codes (use W215 for Nasbar until 6/14), then it's even cheaper. If you're not dead set on a groupset type upgrade, I'll bet you'll be very happy with the new 105 stuff. Even last year's 9spd 105 components are very nice. Plus, you can get them very cheap. I think the best value for the performance you will get. The new 10spd stuff will cost significantly more, but maybe you could just get the new crank/BB combo, which is now the same tech as Ultegra and DuraAce. Finding good bargains on Ultegra is not nearly as easy as it is for the 105 9spd stuff.
FWIW, the new Ultegra triple STI's are really nice. You now can trim out the middle chainring and the large chainring. The large chainring trim is new this year with Ultegra. This is pretty sweet and you can eliminate almost all crosschaining. On the other hand, the Ultegra double keeps the classic crosschaining problem. Yeh, I know, don't ride that way, but that is one fussy double FD.
Common parts that will make your ride like new are cables&housings, chain, brake pads, cassette, and chainrings. Maybe a new BB if it's needed. Get your hubs and headset serviced. BB, headset, and hubs are the most commonly overlooked serviceable items. But, they take a tremendous amount of punishment in normal usage. Make sure you get them checked and serviced, or replaced, as necessary. Assuming your current 105 gear was not abused, then you should be able to get many more good years out of it. It's been said that for $100 a year, you can keep your bike like brand new, year in and year out. I believe that's true.
Good luck with your upgrade!