All DirecTivos are upgradeable. The companies I mention above will sell you a hard drive that you can just drop in or you can find the instructions to upgrade it yourself. I think that one of the companies above (they are blocked here at work) may have the instructions and utilities for a DIY...
No, but you can upgrade to dual 120GB drives internally. What model do you have? This will determine if you need extra parts. Are you comfortable upgrading a computer hard drive? If not, then you may want to look at drop-in upgrades from companies like Weaknees or 9th Tee. -Robert