Well according the the product manule you can have up to 1066 speed memory.
(Section 1-2, page 3 http://www.jetway.com.tw/jw/download_address.asp?downid=6443&proname=HA06&downtype=Manual-English&isbios=
240-pin DDRII Module socket x 4
Support 4pcs DDRII400/DDRII533/DDRII667/DDRII800/DDRII1066
Modules Expandable to 8GB
Dual channel supported
I would say get at least 800 speed. I don't know how much you want to spend but I'm sorry DDR2 costs so much still when you can get faster DDR3 for 1/3 the cost for the same sized stick.