The 4550 comes in two versions (half height and full height) with different amounts of memory.
Either way, it is a genuine single slot card, passively cooled from the factory with no large cooler hanging off the back of it. Power consumption said to be under 25W though I've seen no proper test yet.
Gaming performance should be somewhere between a Radeon 3450 and a Radeon 3650 at a guess (slower than Radeon 3650, on the specs), so certainly not a fast card by modern standards, but maybe "fast enough" for standard HTPC duties and Vista desktop.
As dokokocani said on another thread:
Sounds to be like the perfect Silent PC card for someone like me, who is upgrading from a X1950 Pro.It should not be faster than HD3650, as it has less shadders (80 vs 120) and smaller bus (64 vs 128) and run at an lower speed (600 vs 725mhz). It will likely be lower clocked also. But then, if you think it is sufficient performance for you, go for it.
For the ATI camp, the main debate is probably between this, or a passively cooled 4650 or 4670 which are much faster cards (maybe four or five times the speed, and faster per $ or per Watt). Apart from minor price differences, and the impact on total system power consumption which is important of course in a silent PC, the main difference is physical - seems to me this is a small card, taking less space in your case, keeping its heat away from the sensitive CPU and Northbridge area and generally allowing better airflow. For me, the saving of an hour+ of labour from not having to install an aftermarket passive cooler is also a bonus.
Picture of it and specs (leaked AMD slide):
http://www.dvhardware.net/article29749.html
Rumoured September 25 launch date:
http://www.fudzilla.com/index.php?optio ... &Itemid=34
Pricing expected to be $45-55
http://www.ditii.com/2008/09/09/amd-ati ... ries-gpus/
If anyone has more info, like TDP or real-world performance, it would be great to share it.