If your waiting till August and considering the 9800GTX, you should get the HD4870 instead - nearly the same price I think - just saw some benches of the GTX280 and its an equal of the 9800GX2 at a single core, so we can safely assume the HD4870 should be better than the 9800GTX. However, I expect prices of the 9800GTX to fall so choose between that and the HD4870 - whichever is more value for money at the time.
The 790i SLI board is FREAKING Expensive - if you can afford it, go for it - however, I would recommend a MSI 750i SLI or a EVGA 750i SLI instead.
Note: All this exceeds your budget of 30K easily.