It is commonly said that adding an SSD to your system is one of the single most immediately noticable upgrades you can make.
I agree, and now that SSD technology has greatly surpassed the old days of 'sata controller with usb plugged in', SSD's are basically just as safe to use as regular HDD's.
Personally, I recommend getting 2 smaller capacity (~256gb) drives and putting then into RAID0, effectively doubling your capacity and nearly doubling your speed.
SSD's are stable enough now for RAID0 to be a very low-risk way to give your pc a crazy fast upgrade.