Metaljones,
There are a number of systems available to siphon off or filter out welding fumes. A downdraft system sounds mighty nice, but I am only familiar with some of the portable units (and not many), so I'll keep my comments to those. Given the size of your shop, a portable unit would probably be your best bet for cost and effectiveness. If your shop is heated, it would save you some money to have a portable filtration-type fume extractor that recircuates the air to keep from sucking the heat out of your building during the upcoming winter. Lincoln has a couple of these that may fit your bill. Each are portable floor units. Check the links, if they work.

http://www.mylincolnelectric.com/Catalog/equipmentdatasheet.asp?p=601

http://www.oxarc.com/desc.ydev?prod_id=7653

I have seen both of these models. Our shop has 2nd one listed. It works well, but can be a pain every time you change welding locations as you'll need to reposition the nozzle when you move more than 12". It has a neat feature in that it automatically turns on and off while welding (provided you put your stinger wire through the slot on top), but all-in-all it looks like a mighty expensive Shop-Vac. Granted a Shop-Vac wouldn't have nearly the same effect on your weld plume. I personally like the unit.
Another option, go get yourself one of those fancy self-contained Speedglass Adflo units so you're not inhaling those fumes. Approximate same cost, but kinda bulky and won't help anybody else in your shop.
http://store.cyberweld.com/speedfresair.html

Your local welding supply shop should have these available. I have had very good luck with price and availability from ours, and their customer service is definitely closer than a "dot com".