Is this good???

to run nulldc perfectly.
Intel[R] Pentium[R] III
Mobile CPU 1200MHz
789 MHz 512 MB of RAM

You can probably do it.That's not a lot of RAM, but you'll likely get something. What Operating System are you using, and what video card do you have?

Windows Xp Service Pack 3
Mobility Radeon agp [0x4C59]

It'll be worth a try.Probably work out ok.

Marvel vs Capcom 2 0-28 fps
Project Justice 0-15 fps
Power Stone 30-50 fps

lower settings, display resolution etc.

Nope still really slow.

Either the emulator (incompatibilities) or your hardware then.

The second I saw them, I knew it was gonna be tough...If there are any dreamcast games that are 2-d, you'd be fine...My specs are at least double yours on my laptop, but it can barley do devil may cry 3 on ps2 emulation-and anything like that or gta slows it down to 15fps...If you can chose options that reduce graphic usage, otherwise, it's a dead end as far as emulation goes...But you can probably run 64 games on that power your running-see how you go at smash brothers or f-zero-if they run at 60fps your good :)(Just a sugestion-don't have to take it, but still your pc is a tad weak, sorry mate)

Also, emulators aren't perfect.

I suggest you to increase your RAM a bit... ;)