advice on gaming system upgrade

Hey folks,

Let's say you had the following system:

AMD Athlon XP 2800+
ASUS A7N8X motherboard (for socket A chips)
1 GB Infineon DDR400 RAM (2x512 MB sticks)
Gainward NVIDA GeForce 4 Ti4200 AGP 8 with 128 MB
Terratec Aureon 7.1 Space soundcard
WinXP Home 2002 with SP1

And you had about 300 Euros / 380 US Dollars to spend on upgrading it. And
you wanted to play a mix of games like American Conquest, Mafia, Pro
Evolution Soccer 3, Mediaval Total War.

What you go buy next to get the best performance boost for your buck/Euro?

Thanks for any advice :-)

Cheers, ES