Hi there, welcome to the forums.
Ahh, the question everyone asks. They even made candy named after it! "Now or later."
Hard to say. I'd say if you are going to wait, you'd need to wait for at least 3 months when the AMD 7000 series and Ivy bridge releases. Today's GPU and CPU presentation is quite strong, so it's not a bad idea to build now.
If you're going to wait for HDD prices to come down it could be as long as 6 months.
It really depends on how long you can afford to go without a computer.
If you have a really long time horizon before you care to have a finished computer, you can sit there and monitor shell shocker deals on places like Newegg and TigerDirect and assemble a good computer for like half the price.
You have to be supremely patient for that, though, and you have to have a pretty good idea of what you want as well as a lot of flexibility to adapt to the deals that get offered.
I wouldn't recommend a plan like that to most people, but if you have a "good enough" computer to use now and a lot of patience and time on your hands in the mean time, it could be worth pursuing.
1. 28nm Graphics are not going to be cheap anytime soon.
2. The hard drive shortage is going to extend well in to 2nd or 3rd Quarter 2012. Hard Drive prices may go even higher.
Yes new stuff is on the horizon but a GTX560 ti is going to provide a very enjoyable gaming experience for you. Plus you are going to pay a premium for the new stuff versus the current matured technology which is lined with multiple driver updates.
I think the deciding factor would be the age / capabilities of your current computer.