|M51 is a series, not a specific model.|
Which specific model do you have? - 8xxx ???
It's probably on a label on the outside of the case.
Just about any decent video card will perform better than onboard video.
However, you may need to replace your power supply as well.
E.g. M51 8141 has only a 310 watt power supply.
Your power supply must have at least the minumum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD!)
You can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements.
(This was written months ago:)
If you want to cover any possible video card, a minimum 600 ot 650 watt power supply will handle any current high end video card, or even a X2 card (two video chipsets on one card) or two cards in two slots.
You can usually replace it with any decent standard sized standard ATX PS.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this:
How much are you willing to spend?
Your computer isn't worth a lot.
E.g. M51 8142 Desktop
M51 8??? Tower
If you don't want to have to buy, or get a used, PS with more capacity, you are limited to buying a video card that can be used with a system with your limited original PC capacity.