There is a lot of good information is this review/guide
by Urlyin and I.
The overclocking part starts here: http://www.techpowerup.com/reviews/Diamond/X1900XT/6
It explains all about the 2D/3D modes of the X1900 series, and a lot can be applied to the X1800 series. The guide is a bit out of date now, but there is still a ton of useful tips in there. Perhaps I'll make an up-to-date version and post it in the articles.