If I understood you correctly, you currently are running 4 GB's of 1066 stable. If thats the case, I would just stick with that for now. You will only get a small improvement in performance, maybe 5%.
I'm sure you could use that money in a way that would give you much better return. Right now, as I look at what is available, 4 GB's of anything would be at least $100. 8 would be at least 150. Thats a lot of dough for 5% increase.
Like I said though, you might want to gather some other opinions on the topic. But that's what I would do.