Yes, I was editing the post with my experiments with -gpugrid when you posted. I could get it up to 4096 before the desktop started lagging a little for an increase of ~1 MHash/s. Not worth it in my case, but will probably make a much bigger difference with more powerful cards.

Also, lowering -gpugrid to 256 the hash rate only dropped to ~47 MHashes/s and I could play encoded XviD and X264 content smoothly at full screen, and was even able to join my Wolfenstein: Enemy Territory server and play with my normal maximum settings at 1680x1050 and 125 FPS without a hitch. Hash rate dropped to ~25 MHash/s while playing though.

With poclbm I can reach ~39 MHash/s at those clock settings but at the expense of an extremely laggy desktop. I never use the normal rpcminer-cuda because it almost freezes my desktop no matter what settings I use and always yields lower hash rates than poclbm. Conclusion: rpcminer-mod-cuda increased my hash rate by over 20% while keeping a completely smooth desktop and even allowed me to play my favorite game while still keeping a good enough hash rate for the GTS 450.