If I was in your shoes, I'd split my upgrade schedule: New CPU, MB, and RAM first. Then, probably a 6600GT in another month or two; or, if I decided to mess with video capturing or HTPC stuff, an ATI AIW 9800Pro.
I'd probably think about upgrading the hard drive at the same time as the video, giving a great excuse to do a fresh install with the new video drivers. Newer hard drives tend to use the new FDB (fluid dynamic bearing) technology, which means they run quieter than ever, and they tend to stay quiet over time.
While on the subject of killing unnecessary noise, consider the ~$30 Zalman 7000 AlCu HSF for the processor--it's provides excellent cooling, even with the fan turned way down to "quiet" speeds, and even on the current hottie, the Intel P4 Prescott. If you want to upgrade that now, make sure the revision you pick is compatible with both your current socket and your planned upgrade. (I think the current Zalman 7000 comes with all it needs for compatibility with Socket A and newer on the AMD side, and Socket 478 and newer on the Intel side.)
Also good for killing unnecessary noise: A new PSU. The Antec True-Power line is pretty good (True-Power models have a thermally regulated case fan output, too), as is the Fortron FSP-350 with 120mm fan. Better still are PSUs from Zalman, and the Seasonic Super series, the 80mm fan Silencer and the 120mm fan Tornado.
To kill noise most effectively, fan choice is important, but its also important to have good airflow. You need unrestricted exhaust vents, and plenty of intake area, because low speed fans at lowered voltages don't make a lot of pressure. It's best to cut out any stamped fan grills on the case, and either leave them open, or use an add-on wire grill.
I've done this on my Lian-Li PC60. It took a big leaf of faith to do this to my expensive aluminum case, especially cutting out the integrated grill on the top "blow-hole," but I don't regret it one bit. I was careful to tape off any areas I wasn't going to cut with thick tape to prevent a Dremel slip from marring the finish. (It paid off, because I slipped several times.) I also modified the intake filter housing, and cut away some plactic on the bottom of the case bezel to admit more air.
With my new-found air flow, I was able to lower my fan voltages, ELIMINATE the two front intake fans, and still enjoy lowered case temps. My lowered case temps now prevent the Fan on my Fortron FSP-350 from ramping up except during gaming, DC projects, and other long CPU-intensive tasks. (I've still got some work to do, because I enjoy DC projects--but only if I can't "hear" them running)
I took some other steps to lower my noise and case heat, but the air-flow mods I made have probably had the biggest impact. The second-biggest impact was probably from replacing the Zalman VGA heat-pipe cooler on my 9800Pro with an Arctic Cooling VGA Silencer Rev. 3. The Zalman heat-pipe does cool silently, but it dumps the heat inside the case and up into the PSU, causing its thermal circuitry to increase the fan speed. The VGA Silencer, while not 100% silent, is very quiet on low, and it exhausts the VPU heat outside the case through the slot directly below the VGA card, eliminating the second-biggest source of heat inside the case.
So, split up your upgrade schedule to fit your budgeting, and if you want your next system to be quieter, steps one and two are choosing a MB without an unnecessary north bridge fan, and choosing the right HSF for the CPU. I'm fond of the Zalman 7000 AlCu for the CPU because of its outstanding cooling, quiet operation, and exceptional value. After that, start thinking about unrestrictive case air-flow, low-speed case fans at low voltage, quiet thermal-fan PSUs, a VGA Silencer to exhaust the VGA heat from the case, and quiet new hard drives. (Seagate and Samsung are well-regarded choices, with Samsung being quieter, and Seagate faster--I'm running Seagates myself.)
EDIT: Wow, that turned into a long post--but since you expressed concern about noise, I thought I'd share some of my experiences upgrading with an eye toward both performance and reasonably quiet operation.
EDIT 2: Slightly more concise revision of same post.