Need More Ram For Old Laptop

I have an old HP Presario which is still using Vista Home Premium. Although it is old it still works fine but I want to increase the RAM if possible. It's current memory is 2 x 1gb DDR2 @333MHz and I want to put in 2 x 2 gb sticks. Looking on eBay it seems hard to find the same RAM so I wonder if I can put in memory with a different MHz, and if so would any brand do? I know Vista is not supported but I will only use the machine offline to run a program which is not compatible with my other computers which are all windows 10. Not having done much except swap Hard drives with another laptop I don't want to blow anything up in my ignorance!!

 

Yes you can run the Crucial memory scanner here:
http://www.crucial.com/usa/en/systemscanner
It will tell you the exact type of ram stick that works on the motherboard for that system and how much maximum is allowed.

 

You might also get the RAM modules for a lower price than at HP (they would have them too).

 

I never buy RAM from a system manufacturer. Use Crucial to get the exact type of RAM stick required, and what the maximum amount of RAM is supported on the MB, then simply google and shop around. But given the age of the system, buying a high-end brand is not good value.