Dell's documentation for the Dimension 4550 incorrectly states that the maximum memory is 1GB. Most websites and memory configurators also state this maximum. However, this is only the maximum memory TESTED for the Intel 845PE chipset (the chipset used by the Dimension 4550) around the time it came out. According to Intel, the actual maximum memory for the chipset is 2GB.
Dimension 4550 owners on Dell's forums and the web in general have reported installing and operating with 2GB without problems, although not all SDRAMs are compatible. All this means the Dimension 4550 supports SDRAMs up to 1GB in each of it's two memory sockets for a total of 2GB of memory. However, according to Intel, the 1GB SDRAM must meet certain specifications.
When Dell came out with the Dimension 4550, Intel (with their 845PE chipset motherboards) was not able to TEST the Intel 845PE chipset using 1GB DDR SDRAM memory. While Intel's documentation explicitly states that it is supported, only 1GB SDRAMs with an organization of x8 are specified to work. (For SDRAMs 512MB or less, both x16 and x8 organizations will work.)
Locating compatible 1GB memory:
First, make sure your BIOS is updated to A08. The BIOS version is shown when booting. You can get the BIOS update from Dell's Support download page for the Dimension 4550.
To locate compatible 1GB SDRAM for the Dimension 4550 that meet Intel's specifications, one need only find compatible memory for Intel's D845PESV motherboard, which uses the same chipset as the Dimension 4550. Specifications for Intel's D845PESV Motherboard are: Intel 845PE chipset, 478 CPU Socket, 400/533MHz FSB, ATX form factor.
Any memory manufacturer or memory retailer will tell you what memory is compatible with Intel's D845PESV motherboard. This is easy to do with online memory configurators/locators. Simply enter your system as a desktop or motherboard, maker Intel, Intel motherboard, and model D845PESV. If your Dimension 4550 is a 533MHz FSB (Front Side Bus) use at least the PC2700 memory returned by the configurator (don't use PC2100). For 400MHz FSB use PC2100 memory (faster memory should also work). (If your CPU is 2.2GHz or less you're running a 400MHz FSB, if it's higher you're running a 533MHz FSB.)
Compatible 1GB memory:
Listed below is PC3200 1GB SDRAM memory for Dimension 4550 that should work with either 533 or 400MHz FSB. Note that PC3200 memory supports a faster clock speed than needed, but seems to be more readily available than PC2700 or PC2100 memory at about the same price.
PC2700 1GB SDRAM memory for Dimension 4550 with a 533MHz FSB (these should also work with a 400MHz FSB):
PC2100 1GB SDRAM memory for Dimension 4550 with a 400MHz FSB:
Intel Desktop Board D845PESV - http://www.intel.com/design/motherbd/sv/Intel 845GE/845PE Chipset Datasheet - http://www.intel.com/assets/pdf/datasheet/251924.pdf
Dell Dimension 4550 - Technical Specifications - http://support.dell.com/support/edocs/systems/dim4550/specs.htm
Newegg.com's memory configurator for the Intel D845PESV motherboard, 1GB - http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=1052307858&Configurator=MemoryConfigurator&CFG=CFG003D845PESV&name=1GB