As someone who owns a single bike right now, I have been drooling over two things in the last few months. A new Chorus setup for the current bike (Cannondale CAAD7 with Shimano 105) or a Bianchi Chrome Pista. This really is not an either or question, but which to get sooner. Any opinions would be appreciated.

Get a fixed gear. Spring is the best time to ride fixed. It shapes you up pretty quickly, and there's less maintenance and cleaning to do (around here, the roads can be pretty sloppy in the spring). You may not even want to ride a derailler bike after you get a fixed gear, but you can always upgrade later if you want to. Actually, you should wait for 10 speed before you upgrade.

Upgrades will make you feel better temporarily. Plus once you start you'll go crazy and then realize you spent more on 'upgrades' than a new, similarily equipped bike would have cost. And by then the latest 19 speed freewheel has been developed and you've gotten into a vicious cycle of always needing not the best, but the 'latest greatest' stuff.

Get a fixie. Lots cheaper than another geared bike and adds a lot of variety and benefit to your riding. You won't notice anything significant when you upgrade your geared bike (other than a lighter wallet and some degree of satisfaction), but the fixed or singlespeed bike will open up your eyes to a whole different aspect of cycling.

Have fun.