Short answer: you may or may not see a speed increse
Long answer: Boot time are cut, and you will get longer battery life
the CF bus on these devices is also attached to the RAM in your system so getting a file from the harddrive impacts the entire systems performance (RAM is 100Mhz * 4 byte a sec max transfer rate, whereas CF maes out at 16MB/s (the newer CF+ 3 spec tops out at 66MB/s and supports DMA
shrt bursts should see a speed improvment however i have never seen one of these cards reach the rated speed, the details are hidden away in the data sheets which say that not only is it a theretical max but that it only occurs under certin conditions (ie reading/writeing huge amounts of data at a time
the 8G card would also have a larger cache which would boost performance when dealing with files whos total combined size is less than cache, however this cache helps with overall bus speed if you have read ahead on and at a good value as it prevents the bus from thrashing (which is very bad)
i doubt you would notice a diffrence
IMHO i would stay with the micro drive for swap but if you dont use that then you are better off with the flash card