@adhikari

I'm presuming a cf-card is like what I call an SD Chip. If so, maybe the
following will be of use to you.

I'm using a 2Gb SD Chip myself. I chose to devote only 250m to additional space,
so my total persistent Knoppix 6.2.1 image (on the chip)is only about 1Gb.
This expands to 2Gb in ram. I've got 4Gb of ram, so it fits ok.

This is probably the probably the least workable amount (1Gb total) of SD Chip
memory devoted to Knoppix 6.2.1 that one can get by with. I have a crash now and
then when I get too bold, say with adding little things via Synaptic. In-between
crashes, this little rig works swell.

I expect to expand my additional space (on the chip) to 1-plus Gb next time I
crash and have to restore my SD Chip version of Knoppix 6.2.1. I expect my ram
will handle the expansion, but I'm not sure just how the math works between what's
on the SD chip and what goes and comes from ram. I suspect the stuff on the chip
is compressed about two-to-one, but I don't know this for a fact.

I've been thinking about a larger SD Chip, but because of the math uncertainty,
I'm just not sure how much additional will just fit in my 4Gb of ram. There's
really no other reason I need more ram, so I may not go that way.

Since you already have the 2Gb Chip, I suggest you play around with it for a while
before you get a larger chip. Also, be careful about going above 4Gb, since
there may be some fat32 limitations that make life difficult there.