Aim for 2.5 or more gigs of memory for 32 bit XP or Vista, 64 bit Vista depends on the cpu / chipset. More than 2.5 gigs doesn't buy you much in increased performance by the way over 2.5 gigs but it might be simpler to go to 4 gigs.
Cheerio John
I'm a little confused about getting more than 2MB because I keep seeing comments (not just here) that most programs including 32-bit Vista don't use over 2MB (though I just saw a site that said 3MB is the limit) so why buy more?
gisa,
First I would say to get your system specs, mainly motherboard specs from the computer manufacturer. For example, my current memory (from when I purchased this computer):
Memory 1024 MB (2x512MB) DDR2 PC4200 (533MHz) DIMM
But the motherboard specs for RAM is:
RAM Type (Various) DDR2 PC5300 (667MHz) DIMM
You can see my current memory is slower than the max speed I can use so speed not just memory matters. There's also some info about voltage, but I'm not sure what so I cannot give specific info here, you need a more tech savvy person.
I can say the Crucial is not the only site offering an advisor. NewEgg does, so does Best Buy and others. What is interesting is if you use different advisors, each has different answers. What should be noted here is specs and price of memory, that's where decisions need to be made.
Here's a list of what different advisors tell me for a 1GB stick since I want to go 2MB total (at least).
NewEgg - CORSAIR ValueSelect 1GB 240-Pin DDR2 SDRAM DDR2 533 (PC2 4200) $14.49
Crucial - 1GB, 240-pin DIMM, DDR2 PC2-5300 memory module $15.99
also 2GB kit (1GBx2), 240-pin DIMM, DDR2 PC2-5300 memory module $31.99
Best Buy/Circuit City/Corsair(through Micro Center) - doesn't list my system.
Buffalo (through Micro Center) - a bit too technical for me, I get lost as to whether I want buffered, registered or unbuffered so I can't get past this selection yet.
Kingston - 1GB 533MHz DDR2 Non-ECC CL4 DIMM $12
also 1GB 533MHz DDR2 Non-ECC CL4 DIMM (Kit of 2) $16
also 2GB 533MHz DDR2 Non-ECC CL4 DIMM (Kit of 2) $24
eMachnes (computer manufacturer) - will provide advice through a phone call (for a price when out of warranty) and even offer to sell me memory.
In addition, you could try someplace like PriceGrabber.com for comparison shopping and sort by various methods. For example, above it looks like Kingston is cheaper until you add their $6 Shipping Fee that NewEgg doesn't have. But there is also reviews, some memory like Kingston, Crucial, Corsair have positive reviews, others like Patriot and PNY have none.
Once you look through all this, then it is what items you need combined with ones you like such as price over rating, rating over price, etc.