This is funny, I must be the only one here still rocking Windows XP Pro 32Bit.
I use Lightroom 3 and I think Adobe isn't going to release an update to support the 5D3 in Lightroom 3. I might need Lightroom 4 if i do decide to buy a 5D3...
You could download the free DNG converter (once it's updated) and convert your 5D3 files losslessly to DNG files. LR3 will open DNG files just fine. DNG files have other advantages too. They tend to be a little smaller and there is no sidecar file to keep track of. (Although the sidecar file can be an advantage when backing up because if a DNG is changed you have to back up the whole file again; if a tiny sidecar file is changed it's a MUCH faster process because you don't re-back up the original CR2.)
BTW, I've been Looking at windows 7 Pro 64bit. Is It any Good from first hand experience? XP has been solid for years with me.
XP was a good operating system—the first really stable one for Windows, IMO. Windows 7 is exponentially better in almost every respect. Moving over from Windows XP, you'll probably think that MS moved some controls around to different places for no good reason and, if you're a power user, you'll probably think that MS simplified things to the detriment of users who know what they're doing. But the few trade offs are totally worth it. Just the ability to use more than 3GB of RAM is, by itself, reason enough to switch.