You have done the right thing by asking before diving in and buying! LMCE can be hard work with the wrong hardware, but extremely easy with the right kit.
First your opteron and 2 gig is plenty enough, but check the wiki for a working mobo (most work but some may have probs)
As for your specific requirements
1. HD-DVR - LMCE at the mo only supports burning a blu-ray disc to drive, and playing from there - blu-ray discs are pretty big, so you'll need a bit of space set aside for this! .Also you will not be able to get the disc menus displayed in LMCE (this, we hope, will be rectified in future releases). However, LMCE does support up to 1080p out (so long as you have a good graphics card) - go for a nvidia 7 series as they are best supported.
2. Best generic media playback system!!
3. You need to check the wiki for tv cards that are plug and play. The best ones with an 'av in' for security cameras are the pvr-150 and pvr-500 (the latter will enable you to plug in 2 av inputs). Get a nova-t 500 for digital tv!
Getting your system to interact with a tv can be awkward to set up, but it can be done - you will need a tv with an rs232 port (that isn't specifically for maintainance) but these are limited. Instead you can buy a usb-uirt box to send ir codes to your tv.
Like I said this can be awkward to set up - I just use a fiire chief remote which has a universal ir transmitter so dont need to set LMCE to control my tv