you can swap brands as long as there the same basic make. if you buy 1 with higher clocks then more often than not you will have to use the lower clock settings...
as for the 430 case its gonna be a tight fit and you may have to sacrifice a hdd bay or 2.
for a single card setup your looking at 430+ so add a second your looking at a bare minimum of a 800w psu. so add 200w for an overhead and you have a 1Kw psu for cooler quieter running.
honestly though running 2 of them cards is overkill, there isnt a cpu out that wouldnt bottleneck them even with a major oc.
over all i dont thing that the elite 430 will offer enough air flow to keep your system anywhere near as cool as you would like... come to think of it there isnt many cases that would offer good enough air flow to cool both them cards and the cpu.