How many upgrades before it’s not worth it?

Okay, this is one of those “sell or upgrade” questions, and I’m trying to see if there’s a consensus. Currently, I have two bikes…a commuter (cross bike) and a weekend/fitness/travel bike (henceforth known as n+1). So N+1 is a 2003 Cannondale R400 that I got off CL. It’s in pretty good shape and rides very smooth. I had it overhauled and also put a new chain/cassette on it. All told, I’ve got about $600 into it and I’ve used it for two century rides and a number of shorter rides on weekends, so I think the money was definitely well spent. However, I’m thinking that I’d like to have a more serious/capable road bike with more up-to-date components. The primary weakness is the brifters, which are 8spd Soras. They work fine, but it’s my understanding that I pretty much can’t upgrade any other components until I get 9 or 10 spd shifters on there. So I’m looking at going up to a 105 drivetrain, and possibly also replacing the stem/handlebars and maybe the fork, but that’s a low priority. By the time I do that, I’d almost have spent enough (including my what I’ve spent so far) to have gotten a new bike instead. I figure a 105 equipped aluminum Cannondale runs about $1500.

So what would you do? A) Spend a bunch of money upgrading a bike that fits you well and that you like, or cut your losses and buy a new bike? The one thing I like about option A is that the bike wouldn’t be a cookie cutter bike, it’d be a bit more personalized. Also, gradual upgrades go over much easier with the Mrs. If I did get a new bike, I’d just ride N+1 into the ground and probably pick something up (carbon fiber, perhaps?) next season.

For reference, here is N+1: