You've all heard of Swap Partitions, haven't you? Technically, you aren't *replacing* The RAM on your phone, but you could always create a swap on an external sd, probably one with a higher class, so read/write speeds, and storing/recovering information would be much faster.
Just create one, via CWM, or your pc, and boot into the terminal emulator, and type "swap 0". Or, you could just get an app from the market, if you're too lazy to do all that.
Trust me, your phone does seem a tad faster after doing that.
If you've heard of all this, well, Pardonez-moi.