To do a separation of "boot drive" and "other drive" (meaning, Windows and its essential parts on one disk, and everything else on another disk), you'll have to fuck with Windows configuration more than you're used to. Here are the instructions for what you'll have to do. In the same way, you'd also need to change the location for user profile directories (which contain all manner of files that get made in the name of the currently logged in user, including "My Documents" and all that). This could lead to unwanted fun in a year or three. Some of us would happily do it, come what may.
Anything less than those two steps, however, is not a separation of "boot" from "other", and anything less than the above will fill the suggested 120GB SSD before the end of your PC's life, because Windows updates and personal files alone would fill 120GB fairly quickly in absence of geek-level maintenance.
Then you'll also hit the fact that while putting Windows alone on an SSD will indeed make Windows boot and run faster, Windows is not the slowest and most disk-drive intensive software that you'll install: your games, movies, and downloads (read: "likely everyday use") are. Which is to say that doing the separation will not make GW2 load and play noticeably faster. The separation will make Windows boot very fast, run faster on average, and that puts Windows in a position to hurry up and wait for GW2.exe to read files from the slower disk, because the slower disk will be a bottleneck when GW2.exe wants to load 1 or more GB of highest-resolution textures into memory (RAM) when you log in, switch maps, or start rendering 70 specular Charr simultaneously.
If lightning-fast boot times are not a priority for you, then forget about the Solid State Drive disk for now (cuz newbie hassle and storage constraints on your budget), and buy two or more traditional HDD instead. Try for >= 10,000 RPM motors on both HDD (that's how fast the disk spins), and confirm that HDD, motherboard, and cables are all SATA 6.0Gbps rating (that's how fast data travels from disk to RAM).
Don't do the Windows Registry thing to get a full separation between "boot" and "other". Instead, make the boot disk 1 to 2TB and the other disk >= 2TB. Then, when you install things like GW2, you just tell the installer to put it on the "other" disk. When you set up Steam, the first thing you do is make a setting for its "Steam Library" default location, and that goes on the "other" drive. Etc.
You don't have to trick Steam, when you tell it to use non-default library location before you install the game(s) you want to be there.
The goal is simply to have Disk B (the "other" disk) tasked only with reading data for the game you're playing, while Disk A (the "boot" disk) does whatever the fuck it does. That's the fact that leads to improved performance. Neither has to be an SSD to make that a fact. If you were purposing a single SSD in a position for maximum performance improvement, you'd put it in as Disk B, so that your software runs faster, not Disk A. The next improvement would be to have both disks be SSD.
Also note that, quite relevant to the earlier warning about changing "program files" and "user profiles" locations, then trying to upgrade Windows and installed software on top of that, whether you install Windows 7 or 8 today, you're getting Windows 10 for free later.
Today's Windows 7 Profesional 64-bit sits at about 40GB after catching it up on patches. I have a 4-year used system that sits at 300GB filled. The 300GB could be lower if I wanted to try harder. And fuck... who knows what Windows 10 as an upgrade to 7 or 8 will look like on disk? You can bet that they won't fully account for rare non-standard configurations, though.
When you're done, remember that there's always a honeymoon period with new PCs, just like "new car smell", where everything runs faster than on your old machine, not just due to new and faster hardware, but also due to software not yet having had time to shit all over the file system (then smear it on the walls and draw ASCII penises in it).