You should have looked at Adobe Forums: System requirements for CS5
Now you are in a tough spot. Everything you invest in that Dell is only marginally helpful and will never give you the performance level you had with your HV30 material, which is a relatively easy codec. Your DSLR material is a difficult codec and falls in the most demanding category.
Your best approach is to get a new system. Otherwise get more memory, which will be wasted when you get a new system. Second get a GTX 460+ video card. Don't waste money on a SSD, as it will not help at all to improve perfomance.
Welcome to the forums.
Take a look at the the Premiere Pro BenchMark PPBM5 results. Down near the bottom with a Total Score of 1090 (FB_MAX) you will see a configuration similar to yours. If you then look above at the total score of 979 (John Strickland) you see what adding more memory and an nVidia card can do. You still have only DV/HDV editing capability. Your upgrade plan is hopeless as with your Dell you cannot overclock it to move any higher up the scale.
As Bill and Harm stated, any upgrade to that system would be a waste of money at this point. And although your GeForce 8800 GT is technically a CUDA GPU, the card that you have simply does not have enough VRAM to even enable MPE's GPU-accelerated mode (most 8800 GTs had only 512MB of VRAM). This is because the MPE GPU mode requires a minimum of 765MB of free, unused VRAM (and no, a 768MB card will not enable the GPU mode because Windows itself eats up a few MB of that 768MB of VRAM).
There are at least three problems with your system: the CPU, RAM and the fact that the system is a consumer Dell (which allows absolutely no overclocking of the CPU at all whatsoever) to begin with. Plus, IIRC that XPS 420 uses the old Intel P35 chipset from way back in 2006, which is only PCI-e 1.x compliant (newer systems have PCI-e 2.0 slots). All of those conspire to effectively limit your system's editing resolution to 720x480p videos maximum. And even HDV editing performance is marginal on that system (it is "easier" to edit than DVCPRO 50 on a lesser system only because the resolution of HDV is only 1440x1080 compared to the 1920x1080 resolution of DVCPRO 50).
So in other words, don't waste your money on any upgrades until you earn enough money to purchase an entirely new system.
>sold my Canon HV30's (HDV) and purchased a Canon T3i DSLR
With the computer you have, I think your only option is to find a program to convert your files into something that is easier to edit
Thanks for all of the replies. I really like the benchmark link; extra thanks. I'll just go ahead and use an intermediate codec for now and save my pennies! I've got a $400 gift card coming in the mail in a few weeks. That'll give me a good head start toward a new system. I'll use that benchmark site to plan my system.
Okay okay okay, hold the phone. I just did a little test with Cineform on low, medium, and high, just for fun. All three of them stuttered on full screen playback just like the raw camera file (though scrubbing the timeline was smoother). I found that odd. So I loaded in some HDV footage. Same thing -- smooth until I go full screen. So I loaded some DV footage. Again, same thing. So there's clearly something going on with CS5. Next step is to try another NLE trial and see if that goes any better... this is odd.
Well that was easy enough. I updated my video drivers and voila -- smooth playback, even of the raw camera file. Now it appears editing will be possible after all....