You are not going to gain much in performance by replacing your main drive. Generally if you want slightly faster access time go with a 10,000 rpm drive like a WD Velociraptor. Do not go for a huge drive as it might end up being your garbage collection area and you do not want that.
For the main har drive I will say
OCZ RevoDrive PCI-Express SSD Solid State 120 GBWith a 540Mbps Reading, 480Mbps Writing and a very low latency is like to instal your OS and programs in the ram memory.
If you want to waste money.
I am using Samsung f4 320g, very good speeds. and good price.