Adding additional RAM

[Puddings]

My computer is a Dell Dimension E521.
It has 4 RAM slots.
Atm, I have 3 sticks of RAM in it.
1x 1GB PC2-5300 (333 MHz) by Nanya Tech
2x 512MB PC2-4300 (266 MHz) by Samsung
I also have
2x 512MB PC2-5300 (555 MHz) by Samsung
From what I've read, different RAM types don't really mix too well. If the speed of 1 is lower than the others, than they all run at that lower speed. (or something...)
I was wondering if it would run faster if I had just the two 555 Mhz RAM sticks in there instead of having the 1 GB and 3 of the other ones? Also, if so, will it be a big difference? Should I just use 4 of them anyway for the additional 1.5 GB, or only use the 2?
I know it doesn't affect computer speed directly, but yeah..
I use the computer for gaming and watching movies, mostly. If that helps.
And I know RAM is cheap and I should just buy 4 matching ones and blah blah blah, but I'm just wondering about these since I have them.
Thanks in advance for any advice/comments/flaming.

[Computer_Commando]

My computer is a Dell Dimension E521.
...
1x 1GB PC2-5300 (333 MHz) by Nanya Tech
2x 512MB PC2-4300 (266 MHz) by Samsung
I also have
2x 512MB PC2-5300 (555 MHz) by Samsung
...

According to this:  http://www.crucial.com/store/listparts.aspx?model=Dimension%20E521
the PC2-4300's are not the proper speed for your computer.

[Puddings]

Interesting...coulda sworn those are the two it came with -.-
So basically, considering it says "Although the memory can be installed one module at a time, the best performance comes from using matched pairs of modules" I should put the 2x 512 PC2-5300 (555 MHz) in the first two slots? And considering it says the other pair of RAM is too slow, I should just use the 1 GB in the 3rd slot? >.> Thanks for the link, and response. Appreciate it.

[Computer_Commando]

You can go to www.support.dell.com, enter your service tag and see what RAM was shipped when new.