I've got Noctua's fans and I have absolutely no bad word about them. They are as quiet and efficient as they come. And the price isn't a problem either.
If you're actually out to do some shopping, then this wont help you much, but if you're still deciding what to get, here's what i'd recommend:
Since I dont know the frequency of your 512 sticks, I can't say you need to replace them. Four sticks provides double the memory bandwidth over two, and since 32bit OS's can't adress more than 3.2gb of memory, its fine as it is. Unless its DDR or slower DDR2 RAM.
You're processor, Im going to assume is a pentium D at 3ghz. Replacing that, you probably want to buy a Core 2 Duo E8400 (3ghz, 2 cores, but much more efficient than the pentium D) or a Q6600 (2.4ghz, but 4 cores, and less efficient clock for clock than the E8400). If its gaming, go for the E8400, since only a few games take advantage of 4 cores. Dont even think about futureproofing either; the only way to futureproof now is to get the new Intel processors, which means a new motherboard. Under no circumstances get an Extreme Edition processor, they are simply overpriced. Especially if you look to overclock, in which case a $200 E8400 can be pushed further than a $1000 X6800.
For the Graphics card, I have a ATi HD 4870, and its great. An X2 version is overkill at this stage, and one of my friends had a bad experience with one (Ie, it melted itself) since the reference speed of the radial cooling fan was set so low. I had the same problem with my single GPU card, but I fixed it until the 8.10 drivers came out. Equally, the Nvidia GTX 280 cooks itself too, but its a damnsight cheaper so its essentially half way between the HD 4870 and the HD 4870 X2. If you want to conserve money, HD 4870 is for you. If you want to splash out, get the X2. If you're somewhere in between, GTX 280 fits the bill.
Dont forget the BIOS update. A bios update might allow you to use the Intel chips that are made using 60nm (Q6600) and 45nm (E8400) processors. If not, then you dont have many options to upgrade your CPU and I really would recommend waiting to build a new system. Waiting that is, for the price of the Core i7 CPUs, Motherboards and Memory to drop in price.
Also, if you upgrade that many parts, you may have trouble booting your system. Windows XP assigns values to your hardware, and if it changes too much, it refuses to allow you to boot up. Stupid, you might think, but it prevents valid installations being duplicated by the ARRR pirates.
EDIT: A quick check of your motherboard rules out SLI or Crossfire, since it only has one full length PCI-E slot.