Laptop video cards will always be slower than a desktop version. So the ATI 5870 DDR3 (mobile) vs a desktop ATI 5870 DDR5 can't really be compared. Ultimately, because of the design of a laptop the DDR3 isn't going to be a limiting factor to begin with.
Go for it.
That'll be a good gaming laptop. At 720p the ATI 5870 mobility should run pretty well, DDR3 or DDR5. Again, the laptop's other components probably wouldn't feed it fast enough to make the DDR5 really matter anyhow.