Ah just found the answer on the Ptool FAQ - here it is in case anyone else has the same question:
[b]
What memory card should I use?
SanDisk Extreme HD Video SDHC 30 MB/s, 16 GB
SanDisk Extreme HD Video SDHC 30 MB/s, 32 GB
SanDisk Extreme Pro SDXC 95 MB/s, 64 GB
The above cards have proven to be reliable, consistent from card to card, and fast enough for even the most extreme high bit rate PTool settings. Other cards will work at lower bit rates, but they will hardly be worth your trouble to figure that out. Please just buy one of the above SanDisk cards. The 32 GB card, in particular, is a great value. Most PTool settings are designed for the above cards. discussion
The SanDisk Extreme HD video 30 MB/s cards are apparently being discontinued and replaced with “Extreme HD Video†or just “Extreme†45 MB/s cards. Early indications are that the new cards are at least as good as the old cards in the GH2.
The 95 MB/sec 64 GB SDXC card allows spanning at the highest possible bit rates, whereas the HD Video cards do not. At somewhat lower bit rates, all of the SanDisk Extreme cards will span. discussion
64 GB SDXC cards are subject to an occasional “file limit exceeded†error. Please see the Known Issues section.
The SanDisk Extreme Pro 32 GB 95 MB/sec SHDC card is faster than the Extreme HD Video cards, but not as fast as the 64 GB 95 MB/s SDXC card.
Buy from a SanDisk Authorized Reseller. Fake memory cards are extremely common on eBay, and even on Amazon it's hard to be sure you're getting what's advertised.
SanDisk Extreme Pro 45 MB/sec cards are not recommended.[/b]