that is something no one but you can decide. I do not know your computing needs but i will lay out what you may be looking for. What i would do (what i am doing) is getting 2 vertex 30gb's and putting them in raid 0. However i end up with less storage (20-50% less) because trim is not supported by raid 0 ssd's. If i you are not needing storage (for music, pics, or videos) get 2 vertexes. They are cheaper and i find that the cost/performance is better. You can get 120 gb raid 0 of ssd's for about 340 dollars. Which is all you need if you just have an OS, programs, and even games. You can always add an hdd for storage though! Videos, pictures and music dont need to be speedy unless you are editing those. So, only get enough storage for your os and your programs and your games. In this case 80gb would be plenty if you dont want to worry about raid (and lack of TRIM). Get another hard drive for storage purposes it doesnt matter if its slower (when was the last time you had to wait a few seconds before loading up your favorite song).
Get a second (or your current) hard drive for data/media.
Only use 75-80% of the drive, so the drive can self-optimize. That would be 64GB, plenty for Windows 7, and Office...etc. How much space do you currently use?
Partion the drive (during Windows install), using 64GB as primary partion, leaving 16GB unpartioned and unallocated. Read other posts about this, there is alot. If using Windows 7, let it partion 100MB as "System Reserved", and continue with 64 & 16+/-.