the chrome s2x cards must have the same memory then it works.
so s25 ddr2 and s27 ddr2 will work.
but as you said mixing different memories (ddr2 and gddr3) doesn't work.
the technology is called "multichrome" on s3 cards. it was introduced on the chrome s20 series cards.
it works on every chipset and because of the cards beeing pretty low end it doesn't need any "sli-connectors".
unfortunately only 2 way multichrome is supported. with 3 or 4 cards it doesn't work.
it's always best to use 2 cores at least. with one the performance is worse because this one core also have to "keep" the OS. with 2 cores the second one can "keep" the OS and the other one can be fully utilized for the benchmark.
on some benchmarks it has more influence than on others.
Asus P4G533-LA
http://www.hwbot.org/hardware/motherboard/p4g533la/
Asus P4S8X-X
http://www.hwbot.org/hardware/motherboard/p4s8x-x/
Asus P4S533-X
http://www.hwbot.org/hardware/motherboard/p4s533-x/
Msi MS-7313
http://www.hwbot.org/hardware/motherboard/ms-7313/