Agree a quality one is more than enough. Especially with only a 9800, now if you are planning on picking up a new video card the picture will change... you'll want at least a 530 watt for a singe geforce 260 gtx or better.
Yea its not a bad idea to got with something with more power than you need. Gives you upgrade room. I've always liked having a PSU that is plenty powerful. But I also use my PSU across builds. Some people refuse to do that, but I don't see an issue with it. I have used the same PSU across 3 builds currently.
I have a VERY similiar setup, and I have yet to see my system pull over 300 watts at the wall (and that includes my 24" monitor). Something like this Seasonic unit would be just fine for that system of your's.