so basicly i should stick with x800 and after that it goes like that from less to more powerful (speaking in the same series): GT GTS GTX GTX+... right?
Mostly, but not really. Nvidia naming scheme...
For example, there are 2 kinds of 8800gts, g80 and g92. G92 is far more powerful than g80. G80 8800gts perform the same as g92 8800gs, and below g92 8800gt.http://en.expreview.com/2008/04/02/g80-vs-g92-hi-end-geforce-gfx-cards-round-up.html?page=9
G92 8800gts perform the same as g80 8800gtxhttp://en.expreview.com/2008/04/02/g80-vs-g92-hi-end-geforce-gfx-cards-round-up.html?page=11
G92 9800gtx is g92 8800gts overclocked:http://en.expreview.com/2008/04/03/geforce-9800gtx-review.html/12
G92 8800gt is g92 9800gt renamed, and perform below g92 8800gts, although above g80 8800gts.http://www.overclockersclub.com/reviews/gigabyte_9800gt/6.htm
Get it now?
Anyway, look at benchmarks to decide. Don't depend on naming.