I think 180/200 core/RAM is a reasonable overclock for TNT2 Ultras. I had one back in the day (first TNT2 Ultra ever to come out, the LeadTech TNT2 ultra w/ 5.5ns RAM) and I got it up to 185/205. Hell, with OR without a heatsink it ran 185 core. Heatsink didn't help anything much but to block an extra PCI slot and make for an extra piece of hot metal in my case.
Then I ran it up on a 100MHz AGP bus by accident and its half dead now hehehe.