I'd say turn on vertex lighting. Also all particals, texture, ect on low.
Doesn't exactly help much if you have a dedicated video card already.
Build more bricks.
It is just that your video card isn't that good. I would suggest getting a new PC or at least a new video card.
Video ram? Or Hard drive Ram?
No such thing as HDD RAM.
To increase max bricks you have two options:
1. Get more RAM. Get the right speed and type for your motherboard, and you'll be able to go onto high amounts, BUT:
2. If your video card is weak/you have an integrated video chip, there's a point you may reach where it won't matter how much RAM you have but your video chip will not be able to render all the bricks.
So in other words, get more RAM, and if you really want a lot, get LOTS of RAM and a good video card if you don't have one already.