According to the Apple store 'Configure your computer' extra memory comes as DIMMS and costs: std 256Mb, for 512Mb it's +£50 and for 1Gb +£290. I would guess it only has one DIMM slot.

Base model configured with memory, bigger HDD and keyboard/mouse but no monitor:

Summary

ââ'¬Â¢ 1GB DDR333 SDRAM - 1 DIMM

ââ'¬Â¢ 80GB Ultra ATA drive

ââ'¬Â¢ Combo Drive

ââ'¬Â¢ Wired Keyboard & Mouse Kits

ââ'¬Â¢ 56k V.92 modem

ââ'¬Â¢ Mac OS X

Usually dispatched:

3 - 4 weeks

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Price: £593.20

VAT: £103.81

Subtotal: £697.01 

You can run the OS with 256Mb but I wouldn't want to.