Well, it depends how you use your PC. If you game or use a lot of hardware intensive programs, i.e. Photoshop, etc..., you'll see a huge difference. However, if you just use your PC for Internet based activities and word processing, you'll see a bit of a difference, but not much.
Personally, I'd say go for it, regardless of use. If you can afford it, there's no harm in upgrading and being ready for the future.