Hi, just sorted thsi out myself...
The Dell Dimension 4550 had P4 processors ranging from about 1.6 to 3.2mhz. Less than a 2.6 or 2.8mhz might struggle even if upgraded. The 4550 will support up to 2gigs of ram, but might require a simple free downloadable BIOS update (at Dell.com/support). The ram must be DDR PC2700 (or faster 3200...) LOW DENSITY (no less expensive high density) 184pin. Go to Crucial.com and run the scanner. Their price is $78 currently, but I scored 2gb used on eBay for $40. Until you have 2gb, I would not do anything else.
The 4550 is finicky about video cards. Your 4550 might already have a Dell upgraded nVidia geForce (MX4? 64mb or so), but even the top Dell upgrade is worth replacing. I scored a video card new on eBay for $32.
On the cheap, you could get by with 1gb ram and a 128mb video card, but the bang for the buck will really show up with 2gb and a 256mb video card.
The power supply unit (psu) is a concern. If you have a floppy, 2 CDs & 2 hard drives your psu might be at limit. Adding a video card might overload the psu. The 4550 psu is rated at 250w, but that might be conservative. I'm read that the Dell 250w rating is "continuous" output as opposed to the "peak" that many psu s are rated by... so it might perform little stronger. This relevant because video card manufacturers recommend minimum psu wattage outputs of 300 or 350w, which exceeds the 4550's 250w rating.
With no A drive, 1 HD and 1 CD the 250w Dell psu will comfortably power an worthwhile upgrade on the 4550. Your system has an AGP 4X video slo, but you will want an AGP 4X & 8X, 128"bit" (not "mb", but memory interface compatibility). 64bit won't work and 256bit might be unstable. Next your system will support at least up to 256mb card. Most 512mb cards are too power hungry. Poking around techie blogs I read that the 4550 seems to be friendly to nVidia GeForce video chips/drivers and grumpy with ATi drivers (but many people use ATi cards in the 4550). So that's an nVidia GeForce 256mb 128bit AGP4X/8X. I run a 6200, but they are hard to find in 128bit. I've heard that FX5500, FX5600, 6600, but not sure. Note that there are many manufacturers using the nVidia GeForce chips just to make things more confusing.
Potentially much more important than your upgrades will be having a fresh install of XP and update to sp3. This system will bog with Vista.
Make sure that you have basic free free programs like AVG anti-virus, SpyBot Search & Destroy anti-spyware, SpyWareBlaster (works w/ SpyBot to block some things missed by AVG and SpyBot), remove any unused programs and bloatware (free trials, etc.), run disk clean up and disk defragmenter.
Also consider these two... 1. CCleaner registry cleaner and 2. shutting down any unnecessary process in the "system configuration utility" at START > RUN > 'msconfig" > "services" and "startup" tabs. It's not to hard, but you can mess up here so Windows Help and Google up on it first. A good XP tech can do CCleaner and this in :15 or :20min.
Also, due to their being much less resource intensive, consider Mozilla Firefox instead of Internet Explorer and OpenOffice instead of MS Office (both free).
This will really lean your ol' Dell out. It won't run 2010 games or handle big video editing very well, but it will do many things with adequate speed.
Better than recycling is reusing.
Hope this helps. Ken:^) www.FunnyRealEstateJokes.com