SSDs are a luxury item... its something that you get if youve already maxed out all the other components and still have a little playing money. i dont have a SSD, trying to get my hands on one, but they are very expensive and its hard for me to justify the cost for them. some people swear by them and wouldnt do a build without one.
im trying to get my hands on a 228gig+ SSD, those start at about $250 (used ). im just trying to get such a big SSD because i want to do away with my HDDs completely. a more reasonable approach is to get whats called a boot SSD. which is basically a small SSD, usually around 60gigs, that you put the OS on and maybe a game or 2, then everything else goes on a larger HDD.
All a SSD does is makes things go faster. they dont have any moving parts, so the drive can find the information its looking for a lot faster because it doesnt have to wait for the actual disk to spin up and the finger to find the information. so your operating system is going to open a lot faster, applications are going to open faster and levels in games will load a lot faster.
so if you have any patience, a SSD is not needed... otherwise, go for it!