GTX 275 or GTX 285 or What?

Hello guys, I've just joined here hoping do get a decent answer

I'm planning on a GPU + PSU upgrade quite soon. I want to stick with Nvidia too.
I was initially thinking about a GTX 285 but then came across some benchmarks with the 275 and 285.
The difference was a couple of FPS in most cases but benchmarks hardly ever include my res, 1440x900.
I am just wondering what card I should go for. I want one that will give me very high performance and will keep me running well for the next few years. I can get a 285 for £250ish and I'm happy to spend that as long as it will give me a noticable difference over the 275 which I can pick up for £160.
I overclock too so long as I keep the stock cooling.

PSU wise, I will spend up to around £60.

get a 4890 as they're only ~£140 and are as good as if not better than the GTX 275.

and put the money you save towards buying a nice 750w PSU.

The 285 wouldn't really do anything for you at that resolution.

At 1440x900 I'd got for a gtx 260 core 216 and call it a day. I have a gtx 275 SC and I run 1920x1080 and it laughs at most games on max settings.