Sure it is.
And 4890 OC is a standard ATI SKU, like the XTX was vs the XT. Here's the official line:
"AMD classifies any Radeon HD 4890 card clocked at 900MHz or better as separate product, dubbed the "Radeon HD 4890 OC," although clock speeds on those cards will vary." http://techreport.com/articles.x/16681
Arguing that you "may" be able to overclock the 275 to speeds that beat the 1ghz ATI card is really besides the point. Unless you can buy an overclocked model that beats a 1ghz ATI sku, its really pointless.
Maxed out 275 vs. a maxed out 4890 http://www.anandtech.com/video/showdoc.aspx?i=3575&p=7
As for overclocking a 275 GTX significantly past the point in this review, I doubt it. You may hit ~720 core, or 1700 on the shaders, buts thats what, +2%. Memory is tapped. My GTX 285, for what its worth, maxes out 702/1600/2.7ghz for stability under Furmark, and the consensus from 275/285 owners is that GT200b is pretty much tapped out at ~ 700 core, unless you're volt-modding.
And like I said, 4890's are cheaper. You can pick one up for $180 on newegg right now, and I've seen as low as $150 recently after rebate.
Anyways this is a pointless debate. I've given my recommendation, it wasn't directed at you, nor do you have an argument about anything I've said. You're just trying to show me up since the last time you refuted something you didn't understand, and this isn't really the place for that.