6GB of RAM should be fine. You can always look at your activity monitor under system memory to see what page ins and page outs says. Page outs should be less than 10% to 15% of page ins or you can use more memory for what you run.
Several things you can do. First, make sure that any software listed under login items (under system preferences, users) are at a level to run under Lion. Sometimes when people upgrade, they leave older login items that are not updated to run with the new operating system, and conflicts can really slow the system down. Also look at the bottom of the System Preferenes window for software programs to check, too.
Next, look at your disk usage .... do you have 10% or more of your disk capacity free for systems usage? This can impact performance. You can also run OnyX to clean up the disk if you want. Run the tasks on the cleaning and maintenance tabs. As with any hard drive maintenance software, you should have a backup before using it.
This FAQ on OS X performance will have additional tips.