Bumping your RAM up to 8 GB would have a significantly greater effect on performance than an SSD on an iMac of that vintage and according to
Apple's instructions that upgrade is a cinch to perform. The difficulty might be finding the correct type of memory DIMMs as they are long out of production. Given the internal bus is a 3Gbps SATA rather than trying to replace the internal drive (I agree the replacement job is not for the inexperienced or the faint of heart), you could use an External Firewire 800 enclosure with either a 7200 RPM HD or a 3Gbps SSD. There would be some loss of performance, but the additional RAM would make up for any reduction in disk I/O speed for all but the most demanding tasks.
However, in the end you would still have an obsolete machine with no support from Apple and less and less compatible software.
