The slot on the bottom is just a PCI slot that's universally compatible for smaller and larger daughter cards.
This system would not be suitable for gaming really, gaming requires hi-speed components, and decent motherboards.
It's not just all about the processor and RAM, you have too think about Front Side Bus (FSB) speeds etc.
For a decent motherboard, go for something like an Asus or MSI. The motherboard after all is what will limit too what you can and can not do in the long run.