Microsoft recommend a total size for ReadyBoost of 1-3 times the size of your RAM for best effectiveness. (That said, I'm using 4-times the size of my RAM at home. It won't cause problems, but is unlikey to helping much beyond 3-times - but I've got nothing else I wish to use the SD card for, so may as well use it all up!)
Before you consider replacing your current device, note the minimum requirements for a ReadyBoost device:
- Access time <1ms (ie, all flash devices).
- Read speeds of at least 2.5 MB/s (for 4 KB random reads).
- Writes speeds of at least 1.75 MB/s (for 512 KB random writes).
Sequential read/write speed is not considered. So, check burst read/write speedsof your current device, if they're high above these threasholds it might not be worth the cost of replacing your current device.
Also, note that Windows7 can use multiple devices for ReadyBoost - so instead of replacing your current device you could buy another and use both!
Remember that if you wish to use more that 4GB on a single card it should be formatted NTFS or (preferably) exFAT.