It's pretty difficult to answer that question for you, especially without knowing what you have in your A7 now and what you use it for.
Assuming that you now have a Yonah (CoreDuo), all that you'll really gain from upgrading to merom (Core2Duo) is 64bit support. You'd need to get all new software (windows, drivers and apps) that is 64bit in order to reap the full advantages of this upgrade. Since most software isn't available in 64bit yet, there does not seem to be much to gain (itll depend on what you use ot for as some CPU intense scientific & design apps are avail), but at best only moderate performance increases have been reported in the forums without it. If you are running linux however, there is more software supporting 64bit there and the performance increase should be much more notable.
Personally, I say keep the cpu the way it is and once the warrranty expires, then consider swapping the cpu, unless you really need the extra power and can make use of it. Besides the merom's are still pretty hard to find. Most of the ones floating around (at least the ones that I've seen) tend to be ES (enineering sample) versions and the real merom's out there that I've seen are still too expensive.