1) If the case is about 3ft/1m off the ground it will be exposed to MUCH less dust.
2) Someone said something about zip-ties, that's something which will help with dust accumulation and general airflow.
3) Cleaning the filters and case frequently
4) Try pointing your top fan out so it exhausts air from the case. YMMV there as with my setup this caused temps to increase by 5C for the CPU and VGA, but seriously helped a friend's CPU. He has the same case as you.
5) Another poster mentioned flipping the PSU so the fan is pointed up; good idea there.
6) Aftermarket coolers for your HD4850's should be looked at, perhaps even one for just the top card to help dissapate localized heat pockets and eliminate thermal buildup.
7) You could place a fan on the side if you removed the one which comes with the Antec 300 from the factory. That fan was very useful with above-mentioned friend and his SLI setup. He has dual-slot coolers which exhaust out the rear for both videocards.
8) Purchase a different HSF, preferably a tower-style one with heatpipes and try dual fans. I have found that with an exhaust fan directly behind my Noctua setup there becomes a windtunnel of sorts which helps to establish a steady stream of air through the case.
9) Watercool the whole thing. $$$ but very effective/neato.
10) Purchase quieter fans. I enjoy the Scythe S-Flex fans and Noctua's, but many have had great success with Yate-Loon fans, Sanyo-Denki, or Scythe's Slipstream line of fans. Target RPM would probably be somewhere between 1000 to 1600 RPM, preferably closer to 1600 I'd think for temps. It won't be silent, but should be fairly muted if you stay within that range, and acoustically better with lower RPMs of course. Personally the fans I use are about 1200RPM and they have a low prescence which becomes background after a while. I notice them most once I turn the PC off obviously.
What budget are we looking at? As you can see there are many things with a wide range of prices from free to $$$.
BTW Since you have a fairly free-flowing case, it will scatter noise more readily and given that it is in a corner you also have the effect of this noise taking advantage of the acoustics of being in a corner and spreading out, becoming artificially louder. Moving it from the corner should help a bit.
EDIT: Pics of the side door on the case should help, as well as one with it off showing the fan blades. This will allow us to see which direction the air is flowing and where the fan is blowing the air to in the case. Bonus points if you use a laser pen to determine this and show us a diagram. The laser pen (when the side panel is on the case) can be traced along the edge of the fan blades to highlight the components inside the case, thereby showing where the air is flowing to within the case. You may need to rotate the fan as you do this.
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