I'm in this exact same situation. I'm not surprised others are here with me. While it's true that waiting may get you nowhere but to more waiting and disappointment, I think that MacRumors has that buyer's guide up for a reason.
Let's assume that the guide is correct, and that an update is expected by March 12th or so (calculations may vary by a day), if I buy it on that date I'll be at the height of the technology and update cycle. I won't be expecting an update for at least another 193 days or so. More than half a year is fine with me. Getting burned on a purchase because I didn't wait a few days/weeks is not.
Right now I'm looking at getting a 20" iMac G5, Bluetooth built in, Aiproprt built in, Airport extreme base station, a 1 gig RAM stick from Crucial, and Apple care. I see by the cycles that an update is coming. I don't know what may be updated, but I'd rather get the newest than take my chances.
What could be updated? well this was an okay list:
* A standard of 512 mb memory. You can buy it extra.
* A better graphicscard. Unlikely, Apple does not put super fast graphic cards in consumer macs
* Ilife 2005. You kan upgrade the 2004 version for 20 dollar.
* Faster processor. Oeh, 200 more mhz. Like it would make a difference.
I'd add 8x superdrive (so it can finally catch up with the ancient eMac), built in Bluetooth (and 2.0 at that), built in Airport, 1 firewire 800 port, larger HD at same price point.
I'd really, really like a better graphics card. Something with more than 64MB of video RAM, as I feel that's going to be the bare minimum for Core Image in Tiger (and I'll be doing a lot of Photoshop work on the iMac). If I could get standard 512MB RAM in there I'd be able to have 1.5 gigs when add my 1 G stick. 1.5 seems a little better than 1.25. iLife '05 and iWorks '05 would be amazing to get for free, I'm not too concerned with a faster processor so much as with a faster bus, and even that can be compensated for (somewhat) by a better GPU and more RAM.
I'm fine with buying Tiger seperately, in fact for expediency I'd prefer it.
Does anyone else out there speculate on what's going to be coming with the iMac update?