I know this THG: Gaming CPU Hierarchy Chart chart is geared more toward gaming but it also applies to your situation IMO.
The THG staff recommends that an worthwhile upgrade would move 3 tiers up in chart. The Q9550 is in the 2nd highest tier.
There is no upgrade option from a Q9550.. It is the best processor available for that platform/socket (LGA 775).. For reducing render times even further, you'd need to set up either a render farm (impractical for home or single user scenario) or a dual processor workstation (costly and impractical for learning purposes)..