You need to know the type of ram and the speed of your ram currently installed on your computer's motherboard like SDRAM PC2100 , DDR 400, DDR2 5300. Also, make sure you know how many pins are on your current memory since you need to buy memory that matches your current memory pin count. If you have memory that has 184 pins, buy memory that has 184 pins. The same goes for speed of memory.
You also want to look up how much ram can 1 ram slot can handle since some motherboards can only handle 1 GB of ram while the newer motherboards can handle 2GB or ram per slot.
I recommend you use CPU-Z :
http://www.cpuid.com/ to find out what type of ram you are currently using.
It also helps going to your computer manufactuer or motherboards website and look for the model you own.
If you have the instructional manuals, look in there, too for more info regarding memory.
Corsair makes
http://www.corsair.com/ makes good ram, but their ram is kind of pricey.
I am currently using samsung ram :
http://www.samsung.com/global/business/sem..._DDR2SDRAM.html It currently have been trouble free.
Kingston makes good ram according to the reviews I read online.
1-2GB should be good enough to make Vista run faster.
I'm using 1 GB of ram for my computer and it runs okay.
I recommend getting a flash drive that is "Readyboost ready" since those flash drives are generally faster and will work with Windows Readyboost.
SanDisk, Duracell, Kingston, Patroit, and Corsair make good flash drives.
I am currently using a Duracell 4GB, and Patroit XT 4GB both are fast enough and have plenty of space for my needs.