That is really normal whenever you add RAM and especially if it is a different brand (name on the stick). Since you've added more memory capacity, the program that tests the RAM finds more to do, it has to check all those new memory addresses. That takes time, and when the machine compares your new test results it might be slightly slower. That's normal and even though the test score is lower, your machine should act faster than actually using the paging file less often (now that it has more virtual ram it does not rely on the hard drive page file as often) depending on what programs you use..
The only way to improve your score in your case is to replace all the memory modules with the most expensive ones on the market, and they should all be the same brand name and with the "lowest memory timings". I think however if you compare your test results with others that post their results on the internet you will find that the larger the memory modules, the lower the "test scores" will become.
It's perfectly normal and even necessary if you are running a lot of RAM type programs such as virtual machines or heavy video processing, etc. You can make your machine have a better RAM score by installing smaller, faster RAM modules. In fact if all you are using your machine for is browsing the net and pherhaps an accounting program, that is the best way to go. If you are designing huge spreadsheets, creating video, playing games, running web servers, etc., then the lower memory score is perfectly normal and nothing to worry about.
Vista also uses a "flash stick" ram boost option, although you can probably achieve better results by installing or using a high speed hard drive that does not have your "C Drive" or system files on it, and dedicate part of your paging file (2 to 4 gigs) to that hard drive to improve performance (by using part of a formatted partition and reserving that part through the advanced memory configuration in Windows system settings). You would want to disable (remove) any paging file on your "c drive" and put it on another physical hard drive (if you have two installed). A USB drive will be slower than a native (inside the machine) installed SATA drive).
With the new hard drives (SATA) reading at 3 gig a second, I'm tempted to believe that installing the minimum amount of memory is the way to go, using the fastest dual channel pairs that you can find, and then dedicate a fast SATA drive partition for your page file located on another hard drive. I haven't tested it, I just haven't had the time. Maybe some other Vista fans can lend a hand.