Just get the cheapest, most usable laptop.
- Longest battery life
- Most portable or biggest screen
- Dell still sells systems with XP, at least on their US online store
- The Intel X3100 GPUs are more than enough for Half-Life 1
Macbook Pros are nice (6 hour battery life), but the cheapest model with the 256MB GeForce 8600GT is $2500. If you add on AppleCare and a copy of Vista, it's $3000(!). Way too much for a computer. Right now i'm saving up for either: an Asus G1S, or $1000 of parts for a desktop and an Asus EEE PC.