2 slimline HTPCs - need direction

Hi. I decided to finally retire my old Xbox media centers and replace them with more powerful HTPCs so they could handle the new skins and video files better. I recently finished putting together my two HTPCs and would like some feedback. Both PCs are used primarily for streaming music, sd-video, dvd iso files, pictures, and watching Hulu. I have XBMC, Hulu installed on both PCs and I am able to control them using the Harmony remote controls.

I would mostly like to make sure that the PCs are capable enough and that XBMC is setup properly to make the video look as good as possible since there is an awful lot of standard def video and dvd iso files that will be played. I have a huge DVD collection that has been ripped 1:1 to iso format is being streamed to each HTPC.

Please give me any advice or tips that may make the setup better.

I plan on upgrading the video card in HTPC #2 so it can output HDMI to the TV. I have always been a fan of nvidia but I'm not sure what half height cards are available that would be a good fit for this setup.

HTPC #1 Livingroom:
TV - Toshiba 62" DLP
Pioneer Elite 7.2 A/V Receiver
Acer Aspire AX3200-U3600A
Windows 7 64bit
AMD Phenom X3
4gb ram
320gb hard drive
Onboard nvidia 8200 w/HDMI output
Lenovo N5901 Mini Wireless Keyboard
HP IR Receiver
Harmony One Universal Remote

HTMC #2 bedroom:
Sony 40" LCD
HP slimline PC
Windows 7 64bit
AMD X2
2gb ram
160gb hard drive
Onboard nvidia 6150se (nforce 430) w/VGA output
Lenovo N5901 Mini Wireless Keyboard
HP IR Receiver
Harmony 550 Universal Remote

Media server:
Main PC w/2tb internal storage
Cavalry 4bay E-sata enclosure (4x2tb)

are they bought yet?

if not read here http://forum.xbmc.org/showthread.php?tid=92375

Yes, as mentioned, they are both up and running already. They are both commercial slimline desktops from Acer and HP. Thanks for the link though. It will come in handy when I build one. In the future, though, I think I will go with a nettop since they are much smaller and silent.