Well, I have 3 running computers in the house, and one more in parts waiting for me to get around to buy a new PS. I've got an AMD Duron 950, a PIII 733, and a K6-2 333 running, with a Celeron 466 in parts. The Duron is running on an ECS K7S5A with WinXP and currently has been running for about 3 weeks since I upgraded to a new overclocking BIOS, before that I doubt I had rebooted it in over a month. The PIII is currently the most problematic system I have, but I think it's due to the on-board S3 video, and my younger siblings messing around. I run the K6-2 next to my desk, I use it for an extra browser window, email and SETI, I have it on an Asus P5A and Win98SE, current uptime is 3 months 12 days 5 hours. While I can't claim to have any AMD chips running after 5 years (I killed my 233 last year in an extreme cooling test gone wrong) I also don't have any intel chips running. I do have both of them in the form of 486/Pentium chips sitting on the shelf, I would suppose they still work.
I make no guarantees you'll have such stability, but for anyone to make unfounded claims of terrible instability is irresponsible, EE degree notwithstanding your brother has been misinformed.
I recommend you determine a budget and list the applications you need your computer to perform, and then determine which CPU type performs best for those applications within your budget. But, don't let unfounded rumours or hype drive you to purchase over-priced, or unecessar parts
As for those claiming that a 5 year old system is useless: Please understand that a significant portion of the people that own computers have no need for the latest software. I have friends and relatives running ancient machines, 486s, PIIs, etc. They are perfectly happy too, the computer runs AOL, (or their choice of isp) and they get their email and can browse for cookie recipes. Realistically, those of us who are enthusiasts about computers are in the minority, we might help drive the sales of the latest hardware, but the consumer market is governed by Joe Sixpack who just wants to be able to email grandma and buy cheap airline tickets.
[edit] Gunf1ghter, you must have had some awful video card on that, I ran Unreal1 and for a short time UT on my k6233 with 3dfx Banshee and could get ~30fps at 800x600 high detail in U1 and ~20fps at the same resolution, medium detail, in UT. [/edit]