Why not try it out first before going out and buying a new ps? Sometimes, PS's will surprise you. For instance, I have a 110w ps in my micro-atx case that's running the following:
amd k6-3+ 600 w/fan
256mb pc133
ati rage pro
ibm 120gxp
32x cd-rom
zip drive
awe64
fast ethernet card
56k modem
parallel-port camera/usb printer/usb cf reader/serial-port pda connection
floppy
No crashes and everything runs stable. Sometimes, you never know. Try it out first on your 300w ps. If it works, you just saved yourself some money. If it doesn't, just buy a new ps - shouldn't be hard to swap in the new one.