This depends to be honest...
You may have adequate cooling in the case you have now. Yes, the hot air may be closer to components, but, as long as the fans are in good working order, I don't see why the hot air can't be flushed out in the same manner as in a full system.
The only difference would be the fact theres more space for the air to dwindle about.
Some if not all 8800GTX's have a back blow out area that exausts the hot air out towards the ports at the back, so hot air doesn't dwindle around for long anyway.
The problem I can see though is that the case only seems to have one 120mm fan, and no others, except for the one on the PSU dragging hot air out.
But at least that'd drag out alot of the hot air the systems making, but no cold air would be coming in to cool the system any more...
Does it have a fan mount at the front that can be fitted with a fan of some size or other?
If so, you may still have a chance with the one you already have.
If it doesn't, it does still seem to have some duct, that may work by blowing air in from the force of the fan at the back, though that wouldn't be so efficent, or I wouldn't think.
I'd personally in that situation if a fan can't be fitted, fit a fan and either drill it into the case manually, or celetape it on... its a mod of some sort... and would definetly help cooling.
PSU:
I doubt the power supply your case came with could supply a stable power requirement for the card to be honest.
The 8800GTX is one powerful card, needing a couple of power connectors extra from the one in the PCI-e slot...
I'd advise getting a new one, and a good one at that.
Preferably 500 or more in watts would be sufficent.
Making it a more efficent one will also help cooling in your smaller case, as the less efficent it is, the more heat that will be generated by it, as the energy wasted will just convert to heat.
