Is 1GB + 1GB RAM better than 2GB +0.5GB?

Well, it shouldn't matter TOO much, but using two similar strips of RAM would be recommended.

Just found you the Toshiba Qosmio G25-AV513 has a maximum support of 2 GB of ram so I would recommend buying two 1 GB strips or just use one 2 GB.

You need to have single stick or matched pairs of memory. Which you need will depend on the system and some systems can take either.

If the system can take either it doesn't really matter whether you buy two 1GB or one 2GB stick - assuming that the specs are roughly the same.

Putting the 512MB in with a 2GB stick probably won't work.

However, double check your system specs on the Toshiba site to verify which you need to buy.