Although many say that mixing speed and size of RAM bracket is not a problem, I wouldn't advise it.
1. If everything goes smooth, your whole RAM will work to the lowest speed of the 2
2. It is very possible that your Mobo will be confused and will not be able to operate at maximum capacity if you have different speed/size RAM brackets installed.
3. All RAM are not built equal with different latency time etc... which means that your CPU would have to wait for data to be read/written to/from your RAM a different amount of time and this may cause bottle neck or more serious problems.
If possible, ALWAYS use brackets of the same size/speed/brand.
My gaming machine:
P4 2.8 GHz
2 X 512 MHz PC3200 in dual DDR mode
Intel D865PERL MOBO
integrated 6.1 MAX audio
Logitech duo MX mouse and KB
ATI RADEON 9800 PRO