Well, u didnt mention what is the operating system 4 your bro's PC. So, i assumed he is using window xp. For window xp, is better to run on a system that having more than 256MB ram. On the other hand, for system crashed, it might be the window problem, ram not sufficient, or maybe your graphic card cant support such games. Try check also what version of directx on your bro's system. For what i know, call of duty need to run on directx 9B. U can try update the directx files.
For upgrading, u can get another stick of 256MB/512MB PC133 sdram. Although the mobo can support both sdram and ddrsdram, but only 1 type can be run only. Thats mean either your want to run sdram or ddrsdram. U cant mix both sdram and ddrsdram.
For gaming, although your processor is quite old, but it still can be run. Graphic card and ram plays important role for gaming. For better gaming, u can get some mid-range graphic card.
For Doom3, is really a high requirement games that need high-end graphic card such as Radeon9800 or GeForce5900 only can play smoothly. I've tried play Doom3 using my GeForce4MX440 graphic card, but it quite lag. For HL2, i manage to play.
AMD Athlon 64x2 4000+
2GB Kingston DDRII667Mhz
MSI 256MB GeForce8500GT
Western Digital 160GB SATAII HDD
Maxtor 80GB 8M ATA133 HDD
LG 4163B DVDRW
17" Samsung 793Dfx Monitor
Win XP Pro