Sweet, would I need to reinstall Windows 7 once the beta is over/I'm done with it?
When it comes to testing the OS, it's best to wait for the Release Candidate (RC) version, which is essentially the last step of Beta. Its aim is to give people an idea of what the retail version of the OS will be like, unlike the Beta which is primarily about testing things (which will be broken). The Release Candidate also requires no beta signup and is free to all.
My old laptop ran Vista, and the Windows 7 Release Candidate came out. What I did was partition my hard drive (essentially segment a portion of data away from the main bulk and trick the computer into thinking you have an additional hard drive). Then I downloaded the Release Candidate installer (which is designed to be burned to a DVD and then booted off of), and chose to install it onto my new empty partition. A long install later and it's done.
A word of caution though, drivers will often have compatibility issues when you are using an unreleased OS. They need to be updated to work properly. Sometimes you'll be fine running them under a sort of compatibility mode, but other things may simply not work. I had to wait a while for video drivers to come out for my card (and was stuck in low-resolution limbo until it happened) and the scroll function on my trackpad never worked again after that point. There is also no official support. If you have a problem, you can't call Microsoft or check any official help guides. Your only source of help would be from people who are currently using the OS themselves and know a thing or two about common problems and how to fix them.
After the OS is actually released, you're usually given a long period of time before the Release Candidate stops working. I believe 7 got an entire year. You have that long to buy the official version, and it will tell you ahead of time when the deadline is. When you finally do get the official version, simply delete the partition, stick the actual install disc in, and choose update. My laptop running Vista was able to update directly to 7 and it let me keep everything I still had on it (so you don't need to do a clean install).
If you do get into the Beta for it though, take advantage and report bugs. For the past two OS releases (Vista and 7) people who beta tested the OS and reported enough bugs/solutions received a copy of the finished OS for free.
Oh good, I should be fine then.

Not really sure what x64 does though.

At its most basic, x64 is a version of the OS that allows larger strings of certain data to be handled. What this means for most people is that they can use more RAM. x32 can only handle 32 bits of data, which limits the amount of RAM your computer can actually use to 4 Gigabytes. 64-bit operating systems, on the other hand have a potential roof well above most people's reasonable amount of RAM. In theory, you can get about 128GB of RAM. Windows 7 Ultimate lists 192 GB (but only 16 for Home Premium) If you want to use more than 4 (which most respectable gaming PC's would want), x64 is the way to go. Plus x64 remains compatible with 32-bit applications, though not with 16-bit, one of the few advantages x32 has over x64. You should be fine though, I don't even know if there are very many 16-bit applications left. Are there any?
The downside is that the advantage of more RAM isn't as useful without more 64-bit programs. You can use more programs in a 64-bit OS, but if they aren't written for 64-bit no single program can take advantage of all the RAM at your disposal. Fortunately enough 64-bit programs are becoming more standard. Unfortunately, not so much with games (as people will see with Skyrim)