Internally I assume? If service manual is correct, HB-F1XD has S1985 engine IC inside, meaning it should be pretty easy to upgrade to 512K (using either 4 pcs. 256Kx4 DRAMs, or a 512Kx8 SRAM). Sadly I don't know exact method for this model... maybe someone else does?
could help to you to have only an overview of how to install an internal memory SRAM Into your sony HB-F1XD, the diagrams is for a HB-F9s, notice especially at the end of the post where you can see the installation diagram of AS6C4008 SRAM (Static SRAM 512K x8).
did you get your HB-F1XD 512kb upgrade? if so i have the HB-F1 which is meant to be same with out FDD.. where did you connect the X5 to so you get 512kb.. all pics on the sight are dead links..