Utilizing 1680x1050 resolution?

A 275 GTX wouldnt be a bad choice. That is wher i would be looking.

Mactronix

I have heard a few people say that with 20' monitor 1680x1050 not to go over a gtx 260. If I had let say 30" monitor than a gtx 260 or higher would utilize the GPU better. Is this true?

Depends of what games you play and how demanding you are. For example if you play crysis, crysis warhead, cryostasis, farcry2, stalker clear sky and you want to play with max settings and good fps then you will need two 2xGTX275 or 2x4890. But if you play games like cod, l4d then one GTX275 or 4890 will do the job.

How would running 29800 gtx+ SLI do running crysis, crysis warhead, cryostasis, farcry2, stalker clear sky?

GTX275 < 9800GTX+ SLI < GTX275 SLI

What does that mean?

Long stort short
275 will run what you want and more (slight future proofing )
275 wont give you any admittedly less common these days SLI issues (drivers, compatability)
Oh and reviews say it over clocks like a demon. Better than the 4890.

Mactronix

Which 275 to buy? What mb? What manufacture? etc.....

1:

A GTX 275 is a good choice. Great performance for a relatively decent price.

Going forward, check this page out for more information on various models (middle of the page) and benchmarks (you're going to need to click on the links):

http://www.gpureview.com/GeForce-GTX-275-card-609.html

2:

To clear some things brought up earlier...

GTX275 < 9800GTX+ SLI < GTX275 SLI / What does that mean?

This essentially means that the GTX 275 does not perform as well (is less than) as two 9800 GTX+ in SLI. This also means that the two GTX 275 in SLI perform better (is greater than) than two 9800 GTX+ in SLI.

Another Nvidia flow chart if you are interested...

9800 GTX+ / GTS 250 < GTX 260 < GTX 275 / GTX 280 < GTX 285 < 9800 GTX+ SLI < GTX 295 < GTX 275 SLI < GTX 285 SLI

I have heard a few people say that with 20' monitor 1680x1050 not to go over a gtx 260. If I had let say 30" monitor than a gtx 260 or higher would utilize the GPU better. Is this true?

Better...arguably yes, but that doesn't mean that a GTX 260 is absolutely worthless at 1680 x1050. In fact, as many, including myself, have mentioned, your set-up could definitely benefit from a graphics card that is even stronger than the GTX 260. The main reason includes games, like Crysis and Far Cry 2, that are graphics card intensive. If you want some numbers, play around with what I linked above or try googling for GTX 275 benchmarks.

Which 275 to buy? What mb? What manufacture? etc.....

I wouldn't go with one of the pre-overclocked cards.
Also, the 896MB cards should be sufficient.
As for the manufacturer, it depends on who you trust and if you want to overclock.