I agree with syg00, and will bet that the two RAM sticks are different speeds (eg, PC 3200 and PC 2700) even though they are the same size (512Mg). Ideally, I suggest that both sticks be identical: same manufacturer, size, and speed. Note that's a recommendation rather than an absolute requirement, but mixing different speed RAM together is almost always a sure-fire way to cause problems, because the effective speed of your memory will be limited to the slower stick. In other words, it's sort of like pairing a champion sprinter with an overweight couch potato in a three-legged race - you'd be a lot better matching runners of equal speed with one another.
Anyway, assuming that the RAM sticks have labels, what do they show? If they indeed are different speeds, then if I were you I'd try to return the new stick for one that matches the previous, existing stick, or to just buy a second new stick at the faster speed.
Other comments:
1. Does your mobo support the speed of your fastest RAM stick? According to what I see about your laptop, it can support 266-MHz SDRAM DDR, which is "PC 2100" RAM. If one of the sticks is faster than that, it's not compatible with your laptop.
2. It is possible that the RAM stick is defective. Thy running
memtest to inspect your RAM. The memtest option is often one of the boot options on most distros, so you might not even need to download anything. Just reboot and check out the various boot options, or use a Live CD such as Knoppix, which has it
3. Lastly, it's possible that one of the sticks just isn't seated properly. Recheck them
Good luck with it either way