Deskpro EN - RAM advice?

I almost forgot - this computer came with a single 256 mb RAM card in,
DIMM, 133mhz etc. According to Crucial.com it should only have up to
three 128 mb cards. It seems fine. Can I safely add a second one of
these, 256 mb again.

(I have a 64mb card in a drawer as salvage, but it is different speed,
100mhz - would that spoil things if I added it too? Or that and a
single 128 mb card?)

It is slightly cheaper to buy two 128 mb cards than one 256 - is there
any reason why a single 256 mb card would be better? I will probably
buy a second hand card for £6 from an ebay seller with 100% rep - I'm
aware of pitfalls with dodgy sellers there.

Thanks.