it really depends on what or how much you will install mine is set at 50g/100g or therabouts. to be honest if your using the xp and dont plan to install vista, and your only going to load a few apps on there you could go quite low but really c drive at 30gb minimum is recommended as xp uses the c drive for paging file and virtual memory. imho 30 gb to whatever you desire is cool. there are tools and software that will actually allow you to resize at a later date,
some may say even 30gb is too much as ssds at 8gb fit xp ok, but defragmented files on a hard drive will increase in frequency the smaller the partition is thats allocated to the op system drive.
Also consider how much you will store i.e photos music videos, if its nominal then you could 50/50 split but at the end of the day you will still have access to the lot (minus the hidden 6gb partition for recovery)
Also as a note its best to store all your personal files i.e photos documents on the d drive, as well as any samsung recovery points you create as then in the event that the xp becomes corrupted when you use recovery software you will not lose any of your personal files as they will be safe on drive d (unless drive suffers severe hardware failure.