So I have GC memory card(3rd party, 64MB) and on it there's save data for some of my PAL games. I recently got NTSC-US GC with some games. Would it be possible to store save data for games from both regions on 1 memory card without any problems or should I buy another one?

Maybe you should just try it, unless you think it would be unsafe.

Don't really wanna risk, don't wanna lose my 50 hours casual PSO file

haha, 50 hours :P

It's funny how 50 hours really is a lot, but then you see people on steam with hundreds and thousands of hours and wonder about life :D

Absolutely not.

NTSC-U, PAL, and NTSC-J all have different formatting. If you try to save NTSC-U on an NTSC-J card, you will be forced to format (aka delete everything) before you can save NTSC-J stuff on there. Keep a separate memory card per region, absolutely do not mix

EDIT: All memory cards are region free, but they "become" whatever region your first save data is from. So a first save from an NTSC-J game makes the card "NTSC-J formatted." Overwritting this with a PAL save data erases all saved data and makes the card "PAL formatted." Sorry for confusion