With a 2 ghz P4 just about any modern game is going to be a slideshow at times. You can make it a pretty, soft glowy high res slideshow with a modern graphics cards but anything released in 2006+ is going to target a 2.4+ ghz dual core for smooth gameplay. Read: you've got 1/3 the required CPU capacity, a new GPU will be a great help, but not a cure all.
All your target games use plenty of CPU when you're talking multiplayer.
Save the money and continue suffering until you can justify a complete system upgrade. You'll need about $600US for a capable modern gaming rig:
E2160 + (DS3L or P35-E or Neo2FR) + fan + 2G ram = $200-210
3850 or equivalent: $140-$180
PSU, SATA HD, case, optical drive = $200
Since you have an OEM OS license you'll need to throw another $55 for a vista home or around $100 for XP. Linux is possible for the games you mention but would require an Nvidia card.