DX is a really poorly programmed game. It hogs RAM, and is a dog on any system - not just the low-end ones. On my earlier config with 256 megs RAM, the system would crash out of the game and onto desktop at random intervals. It was very bugging - you had to save every 5 minutes or so.
Now with 512 RAM the game runs a little better, the new vid card lets it flex all the muscle it wants, and it don't crash so often.
If you're not going to move up in hardware, I suggest you install the patch. I don't use it because it adds two undesirable things: Poison gas in the tunnels in the lower levels (first mission before the HK market) - forced to waste and idle time, and it also ensures items remain on dead bodies till you pick them up - these don't remain after the first click if you don't patch it. I like the pressure.
It also alleviates some crashes, though, and does add a few frames of speed specially in no-detail mode.