Dead Battery; Hood won't open.

[luci5r]

After 7 hours of charging, although there is little improvement, it's still pretty dead. The dashboard lit up just a tad and I was able to use the remote key to lock/unlock the door. But couldn't roll up windows or anything; dashboard is extremely dim.

I'm starting to believe this battery might be done. Might need to get a new battery.

Question: Is replacing the battery hard? If I picked up a new battery from a shop, can I replace it myself? Is it basically as simple as detaching the terminals, installing the new battery in place, and putting back the terminal posts? Do I need to worry about anything special?

Another Interstate MTP-91 is around $170 w/taxes and all around here. Unless I should think of installing a different battery.

Suggestions welcome.

Edit:
I'm also finding another battery for $159 locally, it's the Interstate MTP-48/H6. According Firestone here, that's the newer version of MTP-91 and the recommended one for my '05 Boxster. Any advice on this one as opposed to MTP-91?

Thanks.