Stick to the devil I know, Then.
More importantly stick to a budget
If you are only using Word, Email, simple Excel and Web browsing anything you buy will do the job and that has been the case for the last 5 years
Things to consider
1Gb of memory or more (don't think you can buy less than 1Gb)
Hard disc capacity, for you this is likely more about if you have a digital camera the larger the megapixel the larger the picture, but a 320Gb will hold 10,000+ of photo's, if you buy an external 320Gb drive this could double up as a digital photo backup
Most PC's with XP/Win7 slowdown because programs get installed, not used and forgotten, house keeping so to speak will reduce these issues, as will deleting old files/docs and defragmenting the drive once in a while
And remember you can only type so fast, nothing in the computer specs will change this (ok voice recognition maybe)
And don't forget the Ad's on TV from that well known electrical store that quote "Core2duo will make your web browsing faster!!" in a word carp!!