Some good points here.
If you get the upgrade bug, just be aware of what you're in for. Buying a new stem, for instance, might save a few grams, and it might look cool -- but know that the biggest change is going to be in how you feel about your bike, rather than in how your bike feels. Having sexy parts feels good, and it makes upgrading fun. If you're bored and you've got money to burn, by all means, amp up the stem, seatpost, bars, and what not, but don't expect the bike to perform differently (not necessarily, at least).
If you're interested in making truly dramatic changes, Pete makes a wise suggestion -- a good saddle upgrade can add powerful "bling" factor to your ride, and be a tremendous improvement to your experience. Pedals, same thing. The right pedal makes a big difference i how your body and your bike interact, and that's crucial.
If you're set on bigger hardware changes, new wheels are worth considering; they play a big enough role in ride mechanics that the right changeup could equal a distinctly different feel. A good wheel upgrade is also an easy way to save weight, if that matters to you.
Upgrading from 105 to Ultegra is tempting, but not terribly cost-effective in my opinion. The two groups are very, very similar across the board. If you go that route, expect the chief difference to be in terms of your own sense of bike appreciation.
Have fun, and good luck.