Well, the Samsung LCD does 1280x1024, so you just set your Thinkpad (Using display director) to push the external video at 1280x1024 and you should be good to go. If you have it display on both the internal and external display simultaneously at that resolution, then your 1400x1050 display will shrink to 1280x1024 by putting a black border around it.
But, I would
highly recommend reading through the entire article found at X-Bit Labs (and referenced in the T42 FAQ) about LCD panels.
http://www.xbitlabs.com/articles/other/ ... guide.html
I found it really helped me out a lot in understanding the differences between PVA, MVA, TN+, and S-IPS. Once I had a clear understanding of the display technologies behind the various types of LCD panels, and their associated strengths and weaknesses, it made my decision very simple.
None of us can really decide for you, but I know that my reading made me decide to go with an S-IPS panel. The particular reasons I went with it are that it provides a nice view angle (+-178), has full 8-bit color per sub-pixel in general, and the newest generation S-IPS panels also have great response times (mine is 16ms). As a note, S-IPS is what is in the 15" UXGA Thinkpad, though it has a bit slow response time as it is an older S-IPS technology (around 30ms). It is also what is in the Apple Cinema displays. Theirs typically have around a 20ms response time.
Most displays out there are TN+, as it is cheaper to produce. TN+ displays are nearly always 6-bit color per subpixel, for an 18-bit display. And some of the fast TN+ ones don't even really do a legit 18-bit and in fact dither that somewhat. Displays such as the Apple Cinema displays, which are produced by LG in case you were interested, can actually do 8-bit x 3 subpixels = 24-bit color. Thus, they are quite good at reproducing a nice range of colors. The best out there for LCDs.
But, anyways, reading the article will shed a lot of light on many things. If you have questions feel free to PM me.