The monitor would have been something like this HP L2335 LCD monitor from 2004 (this [1] review is from 2005).

According to benchmarks [2], Half Life 2 was pretty vertex and/or cpu bound, so a mid-to-high range GPU ($300-$400) would be able to run at that resolution.

[1] http://www.anandtech.com/show/1729/2

[2] https://www.anandtech.com/show/1546/

CRTs were all 4:3, and had better specifications than the LCDs of the time. Back then LCDs also followed PC standard 4:3 resolution. It wasn’t until LCD tech was cheap enough for mass production in TVs that 1080p monitors became the norm.

Google tells me the Apple Cinema displays came out around this time as well, so surprised there aren’t more 2560 wide users.