I've had
no substantial problems with Windows 7 64-bit and media playback, including for MKVs. There's no need to worry about 64-bit specific filters currently. The standard x86 ones work just fine. I use the filters included in the current public build of CCCP plus CoreAVC Professional on multiple Win7 x64 machines without issue.
There's absolutely no reason to choose anything other than Windows 7 64-bit right now. All versions of Windows 7 will come with both 32-bit and 64-bit versions on the disc, including Upgrade copies. The only possible exception is OEM discs, which Microsoft hasn't announced policy on. However, even if you buy an OEM copy when they're available, the license will be good for both 32-bit and 64-bit versions of the OS (we're just not clear on what will be on the disc itself). We should have those details by the launch, anyway, and you can't pre-order OEM copies.
You might as well go 64-bit now. There aren't substantial problems with it, and then you can upgrade to 4GB of RAM or more. Even if you don't have 4GB now, you might want to at some point down the road, and re-installing Windows then in 64-bit mode would be annoying just to add more RAM.
If you can return the Vista install, just return it. You don't want that. If you buy a pre-built system from most major vendors right now,
you will qualify for a free upgrade to Windows 7 when it ships next month (plus shipping and handling, of course). However, this offer is from "participating vendors only" and likely doesn't include copies you buy from Newegg or Amazon or anything (though it might be worth checking). So, if you already bought it, just send it back when it comes without opening it. (EDIT: Actually, my bad.
You CAN order Vista from Newegg now and you qualify for a free upgrade to Windows 7 via a coupon. So that'd be another way, if you get a coupon like that, just don't bother to install Vista.)
Pre-order Windows 7 Home Premium instead. You can install Windows 7 now (there are plenty of places to get an ISO for free right now, including from Microsoft themselves). You just make sure to install the proper version that you'll want in the end (don't install Ultimate if you're only going to buy Home Premium), and then leave the CD Key field BLANK. Windows 7 will install and run just fine without the CD Key.
Follow these instructions to convert any version of the Windows 7 install disc to a "general version" that will ask you what version you want to install (so that you don't have to install Ultimate, and can install Home Premium or Professional or whatever instead):
http://www.jasdhaliwal.com/microsoft/how-to-install-any-version-of-windows-7/Then, when it asks you to Activate, just skip it or say later. You have 30 days. Then, when you're getting close to the 30 days, just follow these instructions:
http://www.mydigitallife.info/2008/11/06/how-to-rearm-and-extend-free-usage-activation-grace-period-of-windows-7-to-120-days/(This is not a hack or illegal. Microsoft knows about this, has advertised it on their own TechNet blogs, and is fine with it). You can "re-set" the 30-day activation clock up to three times total (so a max of 120 days if you always wait until exactly the 30th day each time).
Either way, if you pre-order, the disc and (more importantly) the serial number will be there on October 22nd, so you won't need anywhere near 120 days. You'll only need to re-set the activation counter once at this point. As soon as the serial comes in the mail, just activate Windows 7 (which you can do at any time from inside the System Control Panel item, under System & Security).
That's how I'm running ALL of my machines right now except for my HTPC (and I'm gonna get to that world-of-hurt upgrade soon enough).
PS. If you plan to upgrade multiple machines all at once when it comes out, don't pre-order now. Wait for the Family Pack upgrade option, available in most countries at launch or very soon thereafter. The Family Pack will include 3 upgrade licenses for Windows 7 Home Premium for $149.99.