From Mechanical Database
Lists of wheel weights:
Forging (including SSF which is a true forging) results in a murch stronger wheel. The wheel can therefore be made lighter than a cast wheel, while still maintaining superior strength. Multi-piece forged wheels have forged centers and spun rims. Sometimse the rims are spun from forged metal, sometimse they are not. Be sure to ask the wheel manufacturer which technique they use to make their rim sections. A good informative video on how wheels are forged can be found on the centerlinewheels.com site, utilizing spin forging.
Casting can be done in a number of ways. The most prevalent and gravity casting (pour the metal into a mold), vacuum/counter-pressure casting (suck the metal up into a mold) and low-pressure casting.
Relative wheel strength are as follows: 1-piece forged, multi-piece forged, die (high pressure) cast, vacuum cast, low pressure cast, and gravity cast. The strengths are the only relative because weight (or actual mass) affects a wheel's strength, as does the actual design. Vacuum (counter-pressure) casting is used almost exclusively by BBS for most of their wheels. Revolution wheels are example of low-pressure casting designs. Rotas are one example of rims that frequently break due to manufacturing quality.
The general rule of thumb is that for every pound of weight that you add in wheel/tire combo, is equivalent to adding 2x that amount of weight anywhere else in the car. This only applies to straight line accelerating and braking. Weight also plays a role in turning (gyroscopic effect) and in handling due to your suspension having to damp all of the road forces. When it comes to wheels and cars, lighter is always better for performance.