The link below will give you the specifications for the amilo pi1536.
http://www.fujitsu-siemens.co.uk/Resour ... 133611.pdf
You will notice that there are three processor options with this machine, which option did you go for when you purchased it ?
If you have not already done so, have you considered fitting more RAM to the machine instead ?
I am not familiar with this particular machine but the specification would seem to indicate that the on board GPU memory is 128MB, which can be expanded to 512MB using shared memory.
It is only my opinion (and as always happy to be corrected) but I wonder if you should look at these options first before changing the CPU.
Learning all the time.