You can have as many hard drives as there are ports on the motherboard for them. Probably your best bet if you need a lot of space is to put four or five hard drives in a RAID 5 array. Since it's within your budget, I would say go with stalin90's build above. I would make the following changes:
- Use this Newegg.com - XFX GX260NADBF GeForce GTX 260 Black Edition Core 216 896MB 448-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card - Desktop Graphics / Video Cards
instead of the 2 GTX 295s that he suggested. This should be easily sufficient for what you need, and getting one GTX 260 instead of 2 GTX 295s will save about $700.
- I would suggest this power supply: Newegg.com - Thermaltake W0131RU 850W ATX12V / EPS12V SLI NVIDIA SLI Certified
NVIDIA QUAD-SLI APPROVED 80 PLUS Certified Modular Active PFC Power Supply - Power Supplies
. You don't need 1000W (in fact, you probably don't need the 850W, either), and this power supply is $70 cheaper after MIR.
- Get 4 of the Seagate Barracudea 7200.11 750GB hard drives, and put them in RAID 5. Even with the cost of 3 extra hard drives, the above changes will save about $500.
Also, you should know that after a point, spending more on the computer is not going to change downloading performance. (And that point is easily passed by any modern computer.) The bottleneck will be the speed of your network, not the speed of the computer.