The Windows 7 RC might be free and almost as polished as the final product, but it'll start booting you off every two hours after March 1, 2010. If you act now, though, you can get a Windows 7 upgrade license for as little as 50 bucks.
Newegg, along with a handful of other online retailers, is offering the Windows 7 Home Premium Upgrade for $49.99 instead of the $119.99 Microsoft announced yesterday. The same e-tailer also has the Windows 7 Professional Upgrade on sale for $99.99 instead of $199.99.
These upgrades will let you step up from a legit copy of Windows XP or Windows Vista. According to Ars Technica, you can upgrade to any edition you like—Vista Ultimate to Win7 Home Premium, XP Home to Win7 Professional, etc. Speaking of Win7 Pro, that edition includes all of the features of Win7 Home Premium. Check this page for a run-down of edition differences.
Whether you end up going with Win7 Home Premium or Win7 Professional, don't wait too long to make up your mind. Newegg says this promotion will end on July 11. After that, you'll have to pay full price for the upgrades. Windows 7 itself will come out on October 22.