Sorry I can't help you with your question, MrApoc, but I was on Newegg.com a moment ago and got blown away by how cheap these (awesome) video cards have gotten. That GT you got and this XT...
are some sweet deals. Anyone looking for an upgrade for around $175-200 will be well advised to hit these up before they disappear. While not nVidia 8800 series monsters, these video cards offer amazing bang for the buck.
Good luck with your oc'ing adventure, MrA. Only thing I can tell you is test in small increments and see what you get. One person reported 594/855 with his Sapphire X1900GT from 512/657... pretty good overclocking.