The card is usually a crippled version just so the top end cards can be kept over priced.
SE is not quite so bad. It stands for Substandard Edition.
Avoid both they are usually crippled way more in proportion than their price is reduced.
XT, GT, GTX, XTX are usually the worthwhile cards. XL is marginally acceptable. These cards usually just have ledd memory and slightly slower clock speeds.
SE and LE they usually do really obscene things like choke a 256 wide memory bus down to 64 bits quartering texture memory bandwidth and performance, or cut the texture processors by 75% or more. They really coke at high quality or with AA and AF turned on.