I really need help.
I have been having BSODs while playing computer games (any) for over 5 months now. During that period I've replaced my old RAM for a brand new one from Crucial. So it couldn't be RAM problem. All drivers (and BIOS) are up-to-date. It's a custom-built PC:

Intel Bay Field D865GBF
Intel Pentium 4, 3200MHz (16 x 200)
NVidia Club 3D GeForce 6600 256Mb
Maxtor 6L200P0 200Gb 7200rpm Ultra-ATA/133
SAMSUNG CD-R/RW SW-240B (40x/12x/40x CD-RW)
SAMSUNG DVD-ROM SD-616T (16x/48x DVD-ROM)
Windows XP Home Edition Service Pack 2
(let me know if you need more info on my system)

These crashes occur randomly. Sometimes they don't happen at all, sometimes 20 times in 1 hour. It doesn't always save minidumps - restarting without the BSOD itself. Or giving that BSOD with only 3-4 lines. Please help me solve this problem.

Do you have onboard sound? Or a PCI Sound card?

021507 fails at agp440.sys and nv4_mini.sys
-that's video driver issue, try updating your Nvidia drivers.

021407 fails at portcls.sys
-that's sound driver issue

021607 fails at nv4_disp.dll
-that's video driver issue, try updating your Nvidia drivers.

Since it's more than one thing and that fact that it's a custom build, you might also have trouble with overheating. Are you doing nay overclocking?

I got the latest NVidia drivers. I'm not overclocking and the temperature is fine. I'm using two USB sound devices - 5.1 External Surround Sound Card SC-5500p and Headset HS-4100 (640U Silverline USB Headset) with the latest drivers for both devices. I have no idea about mine on-board sound. Don't think I have any.