more ram => more transistors => more power draw => more heat generated in the case and power supply
AHAHAHAHAHAHAHAHAHAHA no.
DRAM is mostly made up of capacitive cells.
Furthermore, the extra power draw would be in the order of a few mA ... let's say that each DRAM IC takes up 0.2mA (i'm basing this off the mitsubishi m5m44400-60), then you have 16 ICs to a card, so 3.2mA - the DRAM runs at 5V so using P = I * E we get 5V * 0.0032A = 0.016W - that's right, just 160mW! Compare that the power supply to a LC575 is 40W according to Mactracker ...
(The M5M44400-60 is a 60ns 512kB (1kx4) DRAM IC contemporaneous to the LC5xx series and thus makes a suitable comparison)
I think Bunsen's assessment of the impact of Moar RAM on system cooling is dead-on. Practically non-existant.