Flash drive is a very slow output device. Current commercial USB drive (2GB, 4GB, 8GB or 16GB) performance is range from 10MB/s to 20MB/s.
An average 5400rpm HDD is 45MB/s to 60MB/s (depends on density of hardisk plate).
An average 7200rpm HDD is 80MB/s to 90MB/s (depens on density/plate also).
A high performance 7200rpm HDD will go up to 130MB/s (ex: WD Black edition).
A RAID 0 or 10 HDD setup will almost double HDD output speed.
A SSD (Intel X-25) will perform 3X of normal HDD speed but costly.
A DDR RAM max is 400MHz, DDR2 range from 533MHz to 1066MHz, DDR3 start from 1333MHz.
What all this means is to optimize PC with value for money vs speed, should start from RAM, then HDD.
Had tried ReadyBoost or eBooster with Flash drive before I upgrade my RAM and HDD. Nothing great about it's performance.
Most of the time I will have to wait longer using these booster. Also tried 2 flash drive for ReadyBoost or eBooster, no improvement.
After upgrade RAM from 1GB to 3GB + HDD upgrade to black edition, see great improvement in my laptop.
Con: Laptop battery operating hour drop about 15%.
My conclusion on upgrade will be:
1st Priority -- upgrade RAM until max.
2nd -- upgrade HDD (can using Norton Ghost type of software to transfer all current HDD into new HDD without mess of re-install).