From my experience overclocking is not that hard if you have the right program(tool). I don't have a 9800gtx personally so I can't tell you how hard it is to overclock that particular card. Does anyone have nVedia 9800 card or something similar that they could suggest a good overclocking tool?
And also I don't know how good the stock heatsink on 9800gtx is, but you probably will have to get an after market heatsink to overclolck it well.
Hope that helps.