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

Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&