If you are asking about "dual-channel" mode (slightly speedier memory accesses), then, yes, that eBay module will probably work. PC3-8500 is a standard timing spec and all modules labelled that way should perform the same. But if the memory architecture of the modules are not the same (number of banks or sides) then the memory controller might not be able to handle them in dual-channel mode. It's usually recommended to install matched pairs to be safe but realistically, IMO, the 5 to 10% increase in performance will not be noticeable when surfing the web or running normal office suites. You will probably see the effect when running a benchmark test but I'd be surprised if you detect any increase in frame rates when playing any games or reduction in time when calculating a huge Excel spreadsheet.
ref: http://en.wikipedia.org/wiki/Multi-chan ... chitecture