I've got a Huawei U8150 IDEOS, which only has 256MB of RAM, and the way it struggles I'm sure having more memory would certainly help.
However when I'm comparing various phones to make up my mind on the next one (I'm looking at these ones btw: pricespy.co.nz) I can't help but wonder how much of an extra difference (if any) does having 1,024MB or 768MB over 512MB of the others?