120GB should be large enough for most users for their OS and applications.
If you are looking at SandForce based SSD's, then you have a couple of options regarding user space on the SSD.
For example. both the OCZ Vertex 2 100GB and Vertex 2E 120GB have 128GB of MLC NAND. The 100GB having 28% NAND over-provisioning compared to the 120 E version having 13% over-provisioning. The 100GB version will be a good deal faster once the drive has reached a "steady state" compared the 120GB E version.
I have a 100GB Vertex 2, with my full Win7 install (with all my applications) I still have 59GB free.
Steady state is reached when all NAND in the SSD has been written to once, when after, blocks have to be erased before new data can be written. This has the effect of slowing down the drive. In the case of SandForce based SSD's this slowdown is quite small, and access times are still more than 100 times faster than that VR you were thinking of buying.