It seems that your graphics system is losing the plot somewhere in the middle of the game, but the problem is unlikely to be the fault of the sound card, unless you have installed it with a DMA or IRQ setting that conflicts with your video card. You may just be running out of memory, but without knowing which games you are running I can't be sure about that.
You don't say if the next problem occurs when playing games from a CD-ROM drive. I have seen these symptoms of distortion and popping when the CD drive can't keep up with the action in the game. Usually it's the music. Sometimes the video gets jerky or freezes for a second. You can sometimes improve the performance by installing the game directly onto your hard disk.
The problem with the system clock stopping is usually caused by a flat battery on the motherboard. This battery is used to keep the clock ticking, and store the BIOS settings while you are not using your computer. I am surprised that it seems to have failed already, as the batteries are usually good for three to five years, and your system seems quite new, being a Pentium 100 machine. You'll need to take it to your dealer to get the battery changed, as most of them are soldered in place, and I don't recommend trying to change it yourself.
With regard to adding RAM and a new graphics card, you can do it yourself if you are careful, and make sure that you follow the instructions that come with the devices. However, if you are going to visit the dealer to get that battery fixed, you might as well have them install the new parts at the same time.
- Ian Yates