Memory doubts... simple question but i havent been able to find an aswer

[Petete]

Hi, im new to the Forum. I hope this is the right place to ask this question. If not, i hope the moderator will move the thread to the right place, where i might get an answer.
My question is this:
A friend of mine who has a 2008 24" imac, upgraded his ram memory to 4 gigs.
He wants to sell me his original 1 gig, REALLY cheap... its practically a gift.
But...
I have a mid 2007 20" imac (MA877etc, i think) and im supposed to use a PC2-5300 / 667mhz ram, and the memory my friend is selling is a PC2-6400 / 800mhz.
Can i use this memory on my 2007 imac? or will it make the system "act funny" or something? Keep in mind the i intend to keep the original 1gig (667) that came with my computer and use the new 1 gig to upgrade to 2 gigs ram.
Will it cause stability problems to have a 667 mhz ram and a 800 mhz ram on my imac at the same time?

thanks!

 

[r.j.s]

Dont mix memory speeds, it will cause problems. With prices they way they are, you can get 4GB really cheap on newegg.com