For the uninitiated, HDD (hard disk drives) classically have spinning platters inside. The drives ability to access data relies on the data coming by as the disk spins. SSDs have no platters. They look like RAM (Random Access Memory) or a USB Drive, but much larger and with faster read/write abilities.
What's the big deal? My computer is an older Pentium dual core and still boots up Windows 7 Pro in 20-30 seconds. It opens Word in 1 second. It opens Photoshop in 2-5 seconds. I've never seen a bigger performance boost.
SSDs are expensive, but you don't need a huge one. Mine is 50GB (< $100) and houses Win7 + all my applications. All storage will be done on an external HDD that will be 250GB +.