All that really seems to matter for air-cooling is to keep as much air as possible moving through the case at all times, I've found.
Blowing air in from the rear helps in the sense that the faster the flow of any fluid, the lower the temp, but without exhaust, the overall ambient temp of the case will continue to rise. This is why CPU and GPU coolers always blow
away from the unit- best thing is to get that warm air out of there.
I never understood why ATX cases don't have a fan on top, in front of the PSU... warm air wants to go up, anyway, so that would be more efficient, right?
But keeping components separated helps, too- I keep my two HDDs separated by one slot space, and a good thing, too, as they will get pretty warm. I had a stick of RAM come loose in there the other day (I have no idea how), and when I first opened her up, I touched my master drive and was surprised at how warm it was. Ought to be a heat sink on that puppy.
I've got two 80s in front (intake) and one 120 in back (exhaust) and despite other problems, my first rig does not have temp issues.
I used to have a rear-slot GPU shroud/blower thingy on that machine, but it was a piece of crap. Noisy and prone to hanging up.
A good idea, though- it'd be smart to make all of the new cards double-slot units, with a shell that directs hot air right out the back, instead of down onto whatever's in the next PCI slot.

The new machine will have a 120 in front and one in back, a big Zalman heatsink/fan on the CPU, and the side vent on the case should draw in a little more room-temp air... we'll see.