Let me start by saying that it's extremely difficult to measure how a RAM upgrade changes the performance of any machine. If you draw a chart of how performance changes with RAM upgrades you will get something like this:
As you can see there is a point where increasing the RAM almost does not chance the performance that point is called by many the sweet spot. In XP that spot was around the 2Gb of RAM. In Vista nobody knows for sure yet where is that point but many people believe that it should be between the 2 and 4 Gbs. One of the best articles about Vista Performance that I have found can be read here in case you want to read more about Vista Performance.
Even when for some people Vista Performance Score do not mean anything, I would like to show how it changed with this upgrade from 1 Gb to 2 Gb.
Before the Upgrade:
After the Upgrade:
As you can see it's not that big of a difference. I have to remind that the highest score for Vista is 5.9 so 4.6 is a high score.
The other two tests I ran was using Crystal Mark. Last week Kevin at Jkontherun reported that he did not find any differences between his tests with 1 Gb and 2 Gb using this tool. I do not have an explanation for his case but in my case where Vista has been running with drivers properly configured and installed for more than a month, the results are different.
Before the upgrade:
After the upgrade:
You can notice the difference in the memory score and the total score.
Overall I notice that the whole system is working better. But if you ask, I think that this change is less dramatic from the point of view of performance than the upgrade between 512 MB and 1Gb. But still to early to say the last word about it. One thing is for sure, to have more RAM wont harm in any way your UMPC and the fact that we can use 2 Gb of RAM in almost all the UMPC in the market using Intel Processors is a good news.