Here are some numbers, tested under Windows 7 in OpenGL with 1mb transfer sizes (512x512x4):
Radeon 5850
cpu -> gpu, glDrawPixels, GL_RGB : 0.73s, average: 0.36ms, 2111mb/s
cpu -> gpu, glDrawPixels, GL_RGBA : 0.90s, average: 0.44ms, 2276mb/s
cpu -> gpu, glDrawPixels, GL_BGR_EXT : 0.71s, average: 0.35ms, 2173mb/s
cpu -> gpu, glDrawPixels, GL_BGRA_EXT : 0.90s, average: 0.44ms, 2278mb/s
cpu -> gpu, glTexImage2D, GL_RGB : 1.45s, average: 0.71ms, 1060mb/s
cpu -> gpu, glTexImage2D, GL_RGBA : 1.61s, average: 0.79ms, 1268mb/s
cpu -> gpu, glTexImage2D, GL_BGR_EXT : 1.43s, average: 0.70ms, 1076mb/s
cpu -> gpu, glTexImage2D, GL_BGRA_EXT : 1.78s, average: 0.87ms, 1153mb/s
gpu -> cpu, glReadPixels, GL_RGB : 2.07s, average: 1.01ms, 744mb/s
gpu -> cpu, glReadPixels, GL_RGBA : 1.43s, average: 0.70ms, 1427mb/s
gpu -> cpu, glReadPixels, GL_BGR_EXT : 14.87s, average: 7.26ms, 103mb/s
gpu -> cpu, glReadPixels, GL_BGRA_EXT : 1.44s, average: 0.70ms, 1425mb/s
Geforce GTX 275
cpu -> gpu, glDrawPixels, GL_RGB : 2.13s, average: 1.04ms, 722mb/s
cpu -> gpu, glDrawPixels, GL_RGBA : 1.53s, average: 0.75ms, 1337mb/s
cpu -> gpu, glDrawPixels, GL_BGR_EXT : 1.70s, average: 0.83ms, 901mb/s
cpu -> gpu, glDrawPixels, GL_BGRA_EXT : 1.54s, average: 0.75ms, 1330mb/s
cpu -> gpu, glTexImage2D, GL_RGB : 2.00s, average: 0.98ms, 767mb/s
cpu -> gpu, glTexImage2D, GL_RGBA : 1.94s, average: 0.95ms, 1058mb/s
cpu -> gpu, glTexImage2D, GL_BGR_EXT : 1.60s, average: 0.78ms, 960mb/s
cpu -> gpu, glTexImage2D, GL_BGRA_EXT : 1.13s, average: 0.55ms, 1810mb/s
gpu -> cpu, glReadPixels, GL_RGB : 2.17s, average: 1.06ms, 709mb/s
gpu -> cpu, glReadPixels, GL_RGBA : 2.20s, average: 1.07ms, 931mb/s
gpu -> cpu, glReadPixels, GL_BGR_EXT : 2.04s, average: 1.00ms, 753mb/s
gpu -> cpu, glReadPixels, GL_BGRA_EXT : 1.44s, average: 0.70ms, 1423mb/s
Quadro FX 5800
cpu -> gpu, glDrawPixels, GL_RGB : 3.46s, average: 1.69ms, 444mb/s
cpu -> gpu, glDrawPixels, GL_RGBA : 2.70s, average: 1.32ms, 758mb/s
cpu -> gpu, glDrawPixels, GL_BGR_EXT : 2.93s, average: 1.43ms, 525mb/s
cpu -> gpu, glDrawPixels, GL_BGRA_EXT : 2.76s, average: 1.35ms, 741mb/s
cpu -> gpu, glTexImage2D, GL_RGB : 2.94s, average: 1.44ms, 522mb/s
cpu -> gpu, glTexImage2D, GL_RGBA : 2.70s, average: 1.32ms, 759mb/s
cpu -> gpu, glTexImage2D, GL_BGR_EXT : 2.18s, average: 1.06ms, 706mb/s
cpu -> gpu, glTexImage2D, GL_BGRA_EXT : 1.62s, average: 0.79ms, 1262mb/s
gpu -> cpu, glReadPixels, GL_RGB : 3.34s, average: 1.63ms, 460mb/s
gpu -> cpu, glReadPixels, GL_RGBA : 3.31s, average: 1.62ms, 619mb/s
gpu -> cpu, glReadPixels, GL_BGR_EXT : 3.12s, average: 1.52ms, 492mb/s
gpu -> cpu, glReadPixels, GL_BGRA_EXT : 2.93s, average: 1.43ms, 698mb/s
The first two machines have similar or the same hardware (X58 platform), while the last machine is older and slower (Core 2 Quad).