I'm glad I could help.
What you could do is sell both sets on E-Bay and then when they sell, buy your 8GB then.
I think if you sold them as you bought them (in 4GB lots), you could eliminate or at least severely limit your down time.

After the first set sells, you still have 4GB in your box and can still use it until the second set sells.

Since the price of RAM has increased recently, you might not lose any money on that side of the deal.

My 8GB was $150.00 (U.S.D.) back at the end of October and now I see them selling for $250.

I'm just saying that with a little bit of planning, you could economically do it without taking a big financial hit.

Let us know how it goes, and good luck!