I have been using Hi Flo filters from day one on my 640s mainly because of economics... The OEM spin on filters through KTM are priced around $25 and the paper filters about $10 which adds up to about $35 combined... I buy the Hi Flo filters in bulk and can get 3 of each {6} for the same price... If you are looking to do the same I found that these guys have good prices on Hi Flo filters and will soon have a proper website up:
http://www.classiccycleparts.com/