^seemed to get decent reviews. You can search Newegg for different ones around that price range if you'd like a wider selection.
If your system has SATA II connections, it can use an SSD. If it has IDE still, then I'm not sure, and with SATA I, you might very well be over saturating the connection, which might not even be worth it. Then maybe you should get a VelociRaptor.
I think you should be able to fit Windows 7 and WoW onto there. I'm currently booting Windows 7 from a 32GB SSD so it shouldn't be too big of a problem...at least at first.
The benefits as far as performance are pretty sweet.
Your OS, your games, your programs (that are installed on the SSD) will load almost in an instant. You'll probably be amazed, especially if your HDD is the crappy 250GB 5900rpm 2.5" I think it is, though it'd still be noticeable if your drive's currently at 7200rpm. You know that laggy period in between when you log in and when your programs will actually start running? You can say goodbye to that. Log in, start WoW, and you're in, all in just a few seconds.
That's all maybe 30 seconds after you first hit the power button, depending on how long your BIOS takes to post. At least, if your SSD is as fast as most and is working properly.
How smoothly your actual game runs is dependant on other things, of course. Mainly your processor and graphics card. After the game loads, the only things affected by having a SSD will be things you load from your hard drive: maybe some graphics or objects, and maybe some maps; but then again I don't play WoW and since your folder is so big, I'd assume there's a lot to be loaded from your computer. Those things should load a lot faster.
The problem is that I'm really not sure if you'll have problems with it later on. The main issue with SSDs, aside from cost, is space. Windows 7 will have no problem being installed on your SSD, and combined with your WoW folder, the 2 will probably fill up half of your drive. The thing with Windows, though, is that the more programs you install on it, the bigger the system folders get, even if you're not installing the programs onto your SSD. If you install Windows 7 on your SSD, over time its system folders will fill up with .DLLs, Temp folders, and a whole lot of stuff that will be hard to clean or even locate. Plus, there are a lot of programs that can only be installed on your boot drive, which will take up even more space.
After having Windows 7 installed on my SSD for about 5 months, the only reason it still fits on my 32GB SSD is because I've been moving files to my hard drive while using programs to fool Windows into thinking they're still on my boot drive, a process which is not only tedious, but also causes the programs that use them to slow down since they're on my hard drive and not my SSD.
Do I think Windows will amass 32GB of extraneous files on your boot drive to fill up the remainder of your SSD? I wouldn't think so, but then again I'm not sure what you'll be putting on there, so I'd just advise you to watch what you're putting on that drive.
You'll definitely have more than enough room if you, say, keep your WoW folder on your 250GB hard drive and only keep your OS and smaller applications on your SSD, except that WoW won't run that much faster. Or you could just get an SSD and run WoW on it, while keeping your OS on your HDD, except that then WoW will run marginally faster as the rest of your computer stays sluggish...
But these are all just things to consider. Basically, it all depends on what you're putting in the SSD. Windows 7 and WoW will both easily fit in a 64GB SSD. They will run much, much faster than they do now. Only issue might be space, but you can take care of that on your own. I would just suggest installing your main programs on your SSD, and installing programs you don't need to run as fast on your HDD.
Oh, and I should point out that I'm assuming you're still going to keep your HDD hooked up. I imagine it'll be very hard to fit all your files and stuff onto a 64GB SSD, unless ALL you do is play WoW.