How does the Geforce FX 5900SE compare to the 5900 or the 5900XT?

I think so. It may have been one with a duff fan or summin that got sent back and was fixed.  Though as long as it has its warranty I wouldn't be to worried about it.

Not sure about the SE model.  I know the ATI SE models are not as good as a plain numbered one, i.e. a 9800se is worse than a 9800. I dunno how Nvidia work all there letters at the end.

Why is the Memory clock so low on that GT  ???

Default GT memory speed is 1000MHz.  Ultra speeds are 400/1100 for most though some are pre-clocked, i.e. the XFX at 425/1200 minimum some may come with up to 450 on the core.

Only thing I find strange is even with a GT o-clocked to Ultra speeds they never score quite as high as the Ultras do in the bench tests  ??? Cant work out why as they should be the same. 

I go along with the GT is best buy at the moment, Im just gonna waste my money though and keepin the Ultra.

I thought about that before I replied but that would mean its running at 1500MHz+  ???  I have seen no o-clock on these cards that high.

Dam if I did I would cancel my Ultra order straight away    Think the most I have seen is just over 1200MHz on the .

There is no such thing as a 5900 SE other than a proprietry name branding.

All the nVidia FX 5900 cards share the same exact GPU, the NV35 I believe.

An SE is an XT.

There isn't much performance difference in the 5900 range.

The performance increases from the FX5900XT to FX5900 to FX5900 Ultra to FX5950.

My FX5900XT is a Leadtek A350 TDH and comes with a util to clock it up to Ultra levels. It's my fastest ever card and performs somewhere between an ATI 9600XT and 9800XT if you are familiar with ATI cards.

Here is some research material

http://www.xbitlabs.com/articles/video/display/faq.html