The Intel 945 chipset is not the best video processor to run HD video with. That's probably your problem with the laptop that you tried it on. The processor could be an issue but processing power is not so much of an issue anymore. I would be willing to bet that it's just a combination of a very slow video card, a not-so-fast processor and slow RAM speed. 1.5GB of RAM is plenty to run Vista and should be enough if you are only trying to run video playback by itself but if the motherboard can't access the RAM fast enough then you would see poor performance on games and video playback.
If you wanted to try to get better video playback, I would shoot for a RAM upgrade first. Just keep in mind that your RAM will only run as fast as the slowest stick (if you are running multiple sticks). Crucial is a great place to pick up affordable quality RAM. Unfortunately, you won't be able to upgrade your video card since it's soldered to the motherboard (like every other laptop except for a select few).
Also, what ugottabjoshinme said is not correct. 32-bit Vista will support any amount of RAM up to 4GB. It is not incompatible with odd amounts of RAM. Due to limited addressing space, anything over 4GB of RAM on a 32-bit Vista system will be ignored.
If you need any help or have any questions feel free to PM me. I'm a network administrator so I deal with this kind of problem all the time.