The EPV is a a board with slightly higher core, but rarified memory which overclocks well and starts off at a slightly higher starting point. But they're not cheap.
The SE is similar to the XT but has a slightly faster core (about 10mhz). Which one overclocks varies alot and it would be best to find out from the various owners, I haven't seen much consistency yet from mfr to mfr. Core seems to go well and memory is a crap shoot.
Look at the features provided and the the price an see which one fits your needs. I know that doesn't tell you much, but I haven't seen enough yet. Perhaps someone who has been shopping for one could help you out.
I would suggest checking newegg and pricewatch to start to get an idea of what's out there.
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If you look for a BFG GeforceFX 5900 you'll notice that its the cheapest 5900 card out there even though its a normal 5900 and not the slower 5900XT. Why this is the case I don't know but its deffinitly worth looking into. Its both faster and cheaaper then a 5900XT card. Dunno how good BFG's warrenty is though.
I've got a leadtek 5900lx and it runs at 450/900 stable and is very quiet.
I don't like to push my card but it'll probably go faster if i tried.
I usually leave it at 450/800 which is standard 5900 speed and i'm very happy with it.
If you can get a standard 5900 for less then you'd be a fool not to,it'll probably goto 5950 speeds with a little help.