The 'MB' in for example a "128mb graphics card" is refering to the the onboard VRAM (video RAM). It's used for storing texture data and whatnot in 3D applications. The more VRAM you have, the larger the amount of 3D data it can store, resulting in better image quaity and higher performance*. It's generally much faster than your system memory; In perspective the average PC system RAM runs at effectively, say, 333-400MHz. VRAM runs at (effectively) 1000MHz and above on many modern GFX cards.
*Although the amount of memory a card has certainly isn't the be-all and end-all of your choice. Also, the only game
that currently utilises more than 256mb is Doom 3 (on Ultra quality settings). And then most high-end cards with 256mb can handle this pretty well.
AGP stands for Accelerated Graphics Port. It's a PCI slot on your motherboard dedicated for graphics adapters (brown in colour). AGP 8x is pretty much the current standard, although a new slot type called PCI Express (PCI-E) is fast becoming more and more popular and will no doubt be the standard in 12 months time. Many PCI-E Mobos feature 2 graphics slots, for the use of 2 nVidia-based cards linked together by SLI (forget what it stands for). This isn't the first time PC users have got to use two graphics cards, a company called 3Dfx (who have since been bought out by nVidia) utilised SLI years back, but that's another story. ATI are planning to release SLI cards soon.
512mb cards are due out shortly; 2 of these beasts on SLI will give you a whopping 1gb of VRAM (although this comes at a impractical price, but I can't wait for the benchmarks
). For more info go here: http://www.techist.com/showthread.php?threadid=47834
I hope I've helped a bit :beard: