Upgrading my Ram for Vista

[imported_Vizzini]

A few months I bought a new comp with this:
Corsair Value Select PC2-4200 2GB 2X1GB DDR2-533 240PIN DIMM Dual Channel Memory Kit

Now I am looking to upgrade to 4gb and want to buy a couple more sticks. Checking out the price for the above it is $277 still however the slightly faster speed memory
Corsair Value Select PC2-5300 2X1GB DDR2-667 240PIN Dual Channel Memory Kit
is only $86!

I must be missing something here right? If the pc2-5300 is really what I think it is (slightly faster) is there any reason not to get it? I assume it will only run as pc2-4200 in my machine due to my other ram but I have no problems with that.

very confused,
Thanks

 

[imported_Vizzini]

Well i was searching other sites for the ram I have in my machine and have been seeing them for under $100 so I am thinking that the $277 is inflated for some reason.

So I will go with the PC2-5300. There is no problems mixing the two speeds in my machine is there?

 

[JustaGeek]

There might be!

Your memory must be able to run at 533MHz. Last year I got a 667MHz memory, and it would just not run at 533MHz.

Just want you to be aware of potential problems.

 

[imported_Vizzini]

Thanks, I decided to play it safe and found a place selling for under $100 bucks.