Well, in either case you're basically trying to put performance tires and racing fuel into a Ford Motel T.
Honestly, I;d say save your money and get the new CPU and motherboard sooner.
Any upgrades to that old system is pretty much a waste of money, as far as Premiere CS5.x performance is concerned. Based on the results of a Q9450 (which is a Q9400 with double the L2 cache) system that I submitted to the PPBM5.com results list, a Q9400 or even a Q9450 system (unless the CPU is heavily overclocked) will still be slower than an i3-2100 system with the exact same components (and even at maximum stable overclock, the Q9xxx will still be substantially slower than a basic quad-core Sandy Bridge i5 such as an i5-2400). And even at that, an i3-2100 is still low-end as far as Sandy Bridge CPUs are concerned.
As such, the only improvement that I can recommend for that system at this point is a "regular" GeForce GTX 560 Ti (the 384-core version): Although any performance improvement gained by the faster GPU will still be held back by the Q9400, the GTX 560 Ti can still be moved to a new system should you earn enough money to purchase all new components. Don't go for a GTS 450 as it's nearly EOL, and its 128-bit memory bus will slow down the performance of any future versions of Premiere. The GTX 550 Ti is marginal for CS5.5.
In the meantime, consider transcoding the DSLR footage to a less-compressed intermediate codec such as Cineform (but not DNxHD, which Adobe only supports with a MOV wrapper and thus will invoke QuickTime, which almost completely eliminates the very advantage of 64-bit editing by using only 32-bit processes, thus slowing down performance significantly even with upgraded PC hardware - one of the reasons why Harm Millaard calls it "QuiRckTime" - in addition to the significant gamma shift that QuickTime produces, usually darkening that entire video noticeably). That will make editing less of a burden to that old Q9400 than trying to edit DSLR footage natively. Plus, much DSLR footage has the MOV container around it, necessitating the use of performance-degrading QuickTime if you're trying to edit such video directly.
I agree with both Jim and Randall's comments. In summary, save up for a full cpu/motherboard/RAM upgrade and a decent GPU upgrade when you can afford that expenditure. DSLR is not an easy codec and you will really want a lot more oomph, not just a little bit.
Regarding what you can do for your current system that would improve speed, not cost much, and also be towards something that can be ported forward with future upgrades, why not get a good CPU cooler and overclock your current Q9400? Overclocking really does help quite a bit, and it is almost free.