If you're planning on buying it anyway, and don't plan on waiting in line at the Apple store the day it comes out, maybe you'd like to buy it at Amazon and throw us a referral? They've set up a special landing page for you to get all the details - right over here (The W at Amazon).
Keep in mind this'll only work on Intels - if you have a PowerPC, you're stuck at 10.5 (and probably need to think about upgrading some time in the next 24 months).
I used to build my own, but have just bought the last two outright, largely because you can get ones more than satisfactory for most users cheaply. The only upgrades I've done to my current and previous ones were upping the memory.