I recommend the Scorpion Racing rad guards. A great Canadian company who make awesome rad guards, and great customer support too.
What I like about these guards is that they come with a tab to attach the rad shrouds. With Flatland, etc you have to trim the shrouds to make them fit with the original tab on the rad.. what a pain !! With Scorpion guards, you don't have to modify the shroud at all.
And to top it off, the company has great customer support. I had a brutal crash last year (which destroyed by header pipe) and also bent the shroud tab on the brace (without the rad guard, by rad would have been completely destroyed). I emailed Scorpion Racing to see if I could just get a new tab.. for $12 they sent me four replacement tabs !!
Highly recommended for KTMs... I'm sure they work great on other brands too.
Here is the guard for the WR250:
http://scorpionracing.ca/index.php?option=com_virtuemart&page=shop.browse&category_id=28&Itemid=26
You can also get their products from your local dealer (via the Motovan catalog).
BTW, like Ronzo, I haven't noticed any ill effects regarding cooling when using the full guards.