Not sure about the Antec 300 in anyway, but a good cube case for portability and air flow is the 'AeroCool M40'.
In combination with the OCZ Stealth 700watt PSU it works great for me. Not too heavy, keeps parts cool, fits everything, decent room. I chose it simply because I move my PC using a car a lot, and a tower case typically likes to flip, and laying on its side they stuck off the seat a bit.
Currently I have no troubles (heat/room) with: GeForce 7900GS, OCZ Stealth 700watt psu, IDE HDD, SATA DVD drive, Sound card, Wireless card and a Card reader in this case.
I use 3x Thermaltake 80mm fans as out-take and a Thermaltake 120mm for intake and have no heat issues.
Case has a plastic front, and the top/sides are steel, adding to weight but little sturdier than plastic siding.
One warning though: Not for people with huge hands.