How much of the 1Tb disk space is used?

The spec you've outlined on your existing machine sounds like its decent enough for a home machine that does what you've said. If performance has disimproved then, if you're disk isn't nearly full then defragging the disk might help (depending on your operating system version its likely to be under Control Panel, Administrative Tools, Defragment your hard drive), empty your recycle bin, clear cookies, temporary internet files, etc. and see if that helps a bit. If your drive is more than about 80% full then a cleanup there would probably help performance a bit too (if you clean out unnecessary files, don't forget to empty the recycle bin too). If there isn't much you can clean up then a hard drive upgrade might eek a bit more life out of the machine. After that, checking that all patches, etc. are all up to date would be useful. Sometimes these can be a factor in machines slowing down over time.

Unless you really feel the need for a new machine, I'd say that you could get more life out of what you have with a little bit of maintenance, as above. If its not something you're comfortable with doing yourself then if you have a PC World or some other PC Repair provider then they can probably do all of this for a fairly small cost.

Just a thought in our "built in obsolescence", throwaway consumer goods world