Hai and Welcome to this great website.
I guess you should try DOSBOX, it will solve most of your problems!
Download Link (which you need):-
http://umn.dl.sourceforge.net/sourceforge/...2-installer.exe
Now as you have said about that Pentium III 800 MHz PC, I don't think it will be a good idea to buy that one. Personally I don;t suggest you to go for it. Try DosBox, I am sure it will help you.
But if you are still not satisfies, that PC is good enough for playing DOS Games. Remember, for playing DOS games, you need a Sound Card as a must(not an inbuilt sound card). Then you must know your IRQ's and DMA's addresses. After it, you can play the game. As far as the sound cards are concerned, you sound Creative should do the job!
If you want to go for a cheaper configuration, you can try this:-
Intel Pentium Processor 266 MHz (Pentium II)
16-32 MB RAM
VGA Card
and Sound Card
Thanx