That was fast! It was delivered this morning, and is in my system now.
I installed a Sapphire Radeon HD 4550 - 512MB DDR3 (passive cooling, no fan, so silent) in my SX-2800-03 system (Win 7 X64 Home Premium - upgraded from Vista of the same flavor).
It's a very low power card <25W (or <20W depending on who you consult) - very inexpensive ($46.99 with tax and shipping) and highly recommended overall.
Like I said, I wanted to try the low profile HD 4650, which several people here claimed to be using with success, but it, like the 9600, it looks like a power suck (50W under a load?), and I didn't want to take the chance of hassling with a stressed and blown power supply (the SX-2800 supply is only 220W) over time, down the road.
Also, claims that the HD 4550 would get extremely hot if not vented properly did cause me some hesitation. I was worried about it because the card itself can ONLY be installed in the very top slot (with the VGA socket extender bracket below that). The passive cooling fins face upward (as they should, of course), but the top part of the SX-2800 case is plastic, and I worried about it deforming or melting over time, or causing problems with anything I placed on top.
In reality, I can barely sense ANY warmth on the very top of the case above the card's fins. It's actually cool to the touch after running the card under a very heavy load for several hours. So not an issue at all. The side of the case really is extremely well vented, and you can feel some warmth escaping from the side near the top, just in the area above the (LARGE) CPU fan. So circulation/ventilation is MORE than adequate.
The card itself came with alternate brackets (easy to change) and installed without a hitch. A plethora of updated ATI AMD drivers were available, as well as updates to the Catalyst(tm) Control Center and HydraGrid (virtual desktop layout, very cool) configurator, and the card is performing BEAUTIFULLY.
It is not a heavy duty gaming card, but those are NOT an option with this system anyway, given its power supply limitations. It is, however, MORE than respectable, and adequate for my needs (Adobe Premier, Illustrator, Photoshop, etc., and even some older games).
Anyway, it has also has built-in Blue-ray decoders, for what it's worth. So there's a recommendation for you. I would have no problems nominating this card for Best All-around graphics card for the Gateway SX2800-03.