You will most likely need a memory upgrade as well. Installing the DSL software will probably give you an insufficient memory error if you only have 256M. I've had to upgrade several e-machines for friends who had this problem. They were using XP - I've no experience with ME.
Win ME ... 256MB ... insufficient memory error. I don't think so.
He may not need to install "DSL software". ISP's often want you to believe you need to install special software but it's not always necessary. Just changing Windows connection settings may be enough.
I have my mother's old eMachines 466id (466MHz Celeron, Win 98SE, 128MB) here. Web pages with lots of Flash, javascript, etc. will display a little slower than in a faster machine, but it's still much faster with DSL than with dialup.