Q: See above. Thanks. :)
A: I would go with 1.5GB of 667MHZ RAM. You would notice more of an impact in performance with more RAM rather than faster RAM. Something else to consider is whether you the RAM will be running in Dual-Channel, meaning that they are installed in matching pairs, i.e. 2 x 512MB sticks. This will increase performance. I would just get 2 x 1GB of 800MHZ RAM. (about $40) That would probably give you the best performance for your money. You would also find that 2GB of RAM is enough to do most anything.