Cheap RAM isn't always worth the risk!
Older RAM an be more expensive as the customer base shrinks.
Not worth fitting more than 4 Gig if you are only running a 32bit version of windows 7 as it cant use anymore.
Crucial have a RAM test, they are not the cheapest place to buy from however if the RAM doesn't work when you install it they will replace it.
You could always use Crucial's RAM check to find out what RAM is suitable for your machine then buy it cheaper from elsewhere.