As the q9450 is a 1333mhz cpu and should overclock no probs, and as you appear to not to want to be going above 1600mhz for your choice of RAM, I would recommend either of the following for your two hundred pound budget. You can overclock this RAM much higher than 1600 if you so wish.
4gb 1333mhz RAM and overclocking to 1600mhz (pc12800) - 64bit operating system
2 x OCZ 2GB DDR3 PC3-10666C7 1333MHz Platinum (2x1GB) - total - 187 pounds
or
if you are not too keen on clocking RAM go for
2gb 1600mhz RAM (pc12800) - 32 bit operating system
1 x OCZ 2GB DDR3 PC3-12800C7 1600MHz Platinum Enhanced Bandwidth (2x1GB) - 135 pounds
Personally I would choose 4gb over 2, assuming you are also running a 64bit operating system to fully utilise 4gb of RAM.