If it runs it would be painfully choppy, your pc is just not fast enough. Fallout has deceptive requirements, you almost meet minimum specs, but to play smoothly you'll need a little above that.
To do much of anything with games you'd need a better video card and a larger hard drive. 36gigs is not enough for most modern games accounting for you OS (windows XP), your documents, your pagefile (think of it as extra ram on XP), and some buffer room.
Your video card I've never heard of, making me think it might just be a relabled 6200 with added memory or some other tweak. In any case, it won't run Fallout 3.
That's not to say your computer is useless. Pentium 4's are still good procs, able to run most apps, they just can't do much multitasking. Either way, avoid upgrading (not worth the price to make your pc fast enough), just save for a new pc or get Fallout for a console.