I have been following some of the successful video card upgrades and wanted to add my experience to the mix as it might help someone with deciding to upgrade or not. 

After getting my Samsung 61" HDTV with its 3 HDMI inputs I could not wait to upgrade my HP z556 to Windows Vista Ultimate.  My first attempt to install VMC on top of my current install up MCE 2005 was successful, for the most part.  I used all the drivers I could find for the z565 with no problems.  But the video was less than perfect while watching TV it was often noticeably jerky.  Adding additional memory did not solve the problem and updating the video card drivers only made things worse.

I went back to MCE 2k5 since the wife was getting a bit pissed that she could not enjoy the new TV since VMC wouldn't play nicely.  I thought about different Media Center software or Linux Media Center but I just didn't think any of the versions I saw where as nicely polished as VMC or even MCE 2k5. 

After a day or so using MCE 2k5 after having VMC installed I just didn't like it anymore and it seemed like all the cool third party programs where coming out for Vista Media Center and MCE 2k5 was quickly dying.  So I stayed up late one night and re-installed Vista after formatting the drive.  The installation was not too painful, at one point I had to plug in a USB keyboard while the system rebooted a few times prior to getting any drivers installed.

I again used drivers for the HP z565 to support my hardware with the exception of NVIDIA drivers.  I used what Microsoft had and not the drivers from NVIDIA's website.  The second install was much better and used about 100 megs less memory than the first attempt.  However the NVIDIA 6600 video card just didn't perform as well as I wanted.  I was able to greatly reduce the jerky live TV by lowering the resolution to 1280x728.  But on a 61" TV that is HUGE and I wasn't happy. 

After reviewing several posts on here I was nervous but willing to take a chance at installing a low profile video card in the system.  I picked the NVIDIA geForce 8400 GS low profile card with 256 megs of ram.  This was the entry level card that had full hardware support for HDCP reducing the CPU usage.  I was very worried that this card would not fit in the system or I would have to leave off the bracket.

After reviewing the eVGA geForce 8400 GS http://www.microcenter.com/single_product_results.phtml?product_id=0262595 I decided to visit my local Microcenter and purchase the card still not sure if it would work but with the design of the card I was hoping that it would fit perfectly with a low profile bracket.

The eVGA model was 63 bucks but did not have a low profile bracket and I spotted a chaintech 8400 GS sitting on the shelf just below the eVGA; for 6 more bucks I thought this was a great deal and left the store with the chaintech video card.  Once I got home and begin to install the video card I realized that the low profile bracket was not going to do what I wanted.  In fact it would work at all without heavy modifying.

I took a chance and removed the 6600 and with a pair of pliers I removed the bracket from the 6600 and the full size bracket from the 8400.  The HP custom bracket fit perfectly on the 8400 but covered the s-video port.  this didn't seem to obstruct the bracket and didn't hurt my feelings as I was going to use the DVI to connect this to my TV (DVI to HDMI converter).  I removed the original VGA for the 6600 from the z556 and replaced it with the VGA interface that came with the 8400 card to give me a second method to connect the video card should I want a touch screen in the future.  However the composite and svideo outputs no longer functioned and I just tucked them out of the way so they would not get in the way of any fans.

Bottom line, this was a pretty easy installation, 3 outta 10 in difficulty if you ask me.  I did not have to modify anything with a power took and only needed pliers and a screw driver in order to upgrade the card. 

If your a gamer you might want to go up to the 8500 low profile card.  Also, if you can find one with passive cooling (no fan) I would get that.  I'm sure in a year or so I'll yank that active (fan) heat sink off for a passive cooling option.

I didn't take pictures from beginning to end while installing, I didn't even think about it until after the hardware was in but just to give you an idea of the kind of space you have with a card like this I have included some pictures.