Ah, I'm afraid I cant offer much helpful advice but I will be interested to see what others say because I am on the lookout for effective and relatively economical ventilation myself. I have one of those desks with the built in fan but its worse than useless, I dont even turn it on ay more. The grill that covers it isnt held down with anything it just sits on top of the hole, and it tends to move around when I'm working which is irritating. I intend to get some desktop ventilation, and resurface my nail table to cover the hole.
I remember a while back someone posted on here about a ventilation system which is a separate unit that sits under your desk, and a tube (a bit like the tube that comes off your tumble dryer that you stick out of the window) sits right next to your working area to suck all the dust and fumes directly out of your working area. I've tried searching the forums and I've tried googling it but I cant find anything. When I do find out what it is I will be investing in it because it sounds great.