Just a professional systems builders prospective
I recommend dell for 'soccer moms' that use it for browsing and catching up on the soaps

. Gatway uses more mainline components that are replaceable such as power supplies and such. Most of the dells use propriatery parts that can only be replaced by dell. Typically if your dell dies this is what happens.
1. Call tech support and they run you thru a checklist. Checklist proves there is a hardware problem.
2. Part gets courrier deliverd from nearest part center, here in pa its' newark nj or phila pa.
3. courrier delivers part so you call dell back, tell them part or parts arrived and they dispatch repair tech who is generally a contracted PC repair person.
4. Repair person arrives, fixes unit and you are back up and running, generally takes 2-3 days tops.
5. You are too far away for the above solution or you bought one of the cheaper desktops and it's not worth it for dell to go to all this trouble so they have you ship it back and turn it around in 5-8 days. That is unless you bought 50 of these systems, then it's a different story
On the other hand gateway has mainstream parts and if you are in any way able to turn a screwdriver will drop ship you the repair part overnite in a prepaid return package which is handy for infant mortality failures like HD's and processors and other electronics.
I, as a system builder, tell customers that want me to build a custom box that I cannot beat the prices on the lower level dells and gateways, having to buy the OS is what kills that price difference.
For a general machine an Asus or intel 845 board with onboard gig lan/vid/sound and a retail boxed 2.8ghz P4 with 512mem costs me ~$452, OS takes it to almost $600, add a case, optical dvd burner, SATA HD, floppy, extra case fans and the ball park is around $850, then I tack on a 10% builder fee and show the customer the invoice for the parts I used. The customers I get and want are ones who want performance, reliability and support local business's. Custom builders like me will go the extra yard and use special cases, colored even with color matched keyboards, mice and monitors, we also build 'quiet computers' that you need to look at the lights to see if it's on

I REFUSE to build cheap crappy boxes that have my name on them. Theres nothing that makes my day than another tech saying 'wow, thats nice'.
Plus the better local system builders like me also go the extra mile and generally will deliver or spend the time showing you the features, not to mention are generally a quick phone call away and can respond quickly, not call forwarded to Calcutta india.
Problem is you are pushed for time and finding a really good builder can be hard and one who uses brand name parts instead of mystery boards even harder. There is enough talent on this site that we could put together a dozen different hardware configs that more than suit your purpose and actually building one takes about 30 min once you get all the unpacking done, it is the software install that takes the time. Another 30 min to fold all the cables too to get them out of the airflow.
Good luck with whatever you decide.