Bike Upgrade Questions

After reading the tri v. road bike thread here I am now leaning toward sticking with my Specialized Allez but I am still trying to decide on a few things.

1. My bike is about a 1999 model but I love the way it rides, after reading some online reviews of the 2005 Allez Elite Triple I am questioning if I want to sell my 99 and buy a new one or if I would be better off upgrading mine.

2. If I decide to upgrade the one I have what are the biggest bang for the buck upgrades?

I am a road rider originally who this summer and last has caught the tri bug so I need something versatile. I am stepping up to Oly distance Tri's next year but will still do at least one century and a few other rides through the year.

Any thoughts on just how much of an upgrade shelling out the $1500 or so it would take to get a new bike would be?

I am also interested to see which upgrades you guys think are the best ones to make since besides a few minor things my Allez is pretty much stock still.