If you have got time (and can be bothered) you could put your Smartfan 2 on your current heatsink- place the generic one on your plexi/case side and duct it into your new SF2 and H/s combo- should take quite a bit off your temps.
dont know how you can make a ductwith a plexi side- i'll have a think....
[light bulb flashed above head]
instead of using cardboard (as many people do if they are without window), use a plastic cup (generaly see-thru) and put a couple of LED's on it, might give a cool patern if the cup is slightly crumpled/has pattern in it. [/light bulb flashed above head]
this will give you time to save up for a decent heatsink- so you dont need to buy it all at same time or can wait a while and get a more expensive one.
Price is NOT an indicator of effectivness of a product