Part of the reason the Galaxy phones are so smooth is the phone reserves a 128 MB chunk of ram for the OS to use as a virtual drive, leaving 384 MB usable. No, you can't load any thing in to that chunk of RAM, but you won't get your phone to lag because the memory is full. I think it is a good trade off.
I would rather they had added another 128 to the phone, but it is what it is. I haven't run out of RAM in the D1 yet, so I'm not terribly worried I will fill up the Fascinate. With a 2.2 upgrade, it will matter even less, as you can keep more stuff on the SD card.
YMMV, void where prohibited, etc.