I just switched. Bought a 2.66, have upgraded the RAM to 7gb, also bought and installed the XT1900 card (installed the card myself, it was very easy). I have also add 3 hds from them PC, 2 500gb and one 750gb.
From a speed stand point I am happy. I had a Q6600 running Vista right before, and I can't say that this machine feels slow in comparison. The only places it does feel a little slower are when playing WMV through the divx player but that seems to be more of a software/codec issue as video that opens in quick time is quick.
I know that new ones may come out in the next few months but I did not want to wait and have no regrets of not doing so.
If you really want the $2499 model with the XT1900 the thing to consider is that when you order it you will get credit for the removal of the Nvidia 7300 car vs if you do it at the store you total out of pocket will be a little more, but you will have instant gratification and you could sell the 7300 on ebay for around 65 or save it if you ever wanted to run 3 monitors.