You don't say exactly how your iMac is equiped but going from a HDD to SSD may not get the performance increase you are lookng for. You should be using at least 4 GB ram with LION even though the sepcs say 2 GB. A faster spin hard drive, such as 5400 or 7200 rpm may be enough for what you want to achieve. A 2007 Core 2 Duo is going to be significantly slower than the quad core machines today so you should not expect performance close to today's hardware.
Best approach for now is make sure you have enough ram and a faster hard drive, SSD is very expensive.