After testing Jaguar on some machines (which support or unsupport QE), I got some suggestion.
If you are satisfied with Mac OS X 10.1 now, you don't have to upgrade your system. Because Jaguar doesn't slow down but also improved abit. If you have some little money, you could wait until it is finally released and see if you are really needed to buy a new one.
If you are not satisfied with Mac OS X 10.1 now, save and save and save more money until late-Summer, then get a new machine comes with a release version of Jaguar! Apple has been doing her best to make OS X running better on older machines... But of course, make any common sense, how iMac 233 with 128MB can run OS X on the fly with 1024x768 and millions of colour??
If you are rich, just buy the latest Pro-machine... i-machine won't stay long for long term user... I meant it... The new iMac is fast... But next year, more 3D games is out and you will find that GeForce2 MX won't enough for you... Pro-machine can at least have a chance to upgrade their graphic card!
I hope this helps.