I'm building a server (web server, ftp server, etc) using a Q6600.
Which storage solution best serve my needs, WD 750 gig drive or raptor?

The raptor is faster, the 7200 drive has more capacity.

100% agree, quick access times definetly help. and for a performance server, the raptor would make more sense too (since that seems is the point of it, because there are larger hdds than the 750GB out)

You could get two 300 or 400 gig drives and do a raid for performance/integrity... depending on the board you have. Cheaper then Raptor, and faster I believe.

the only problem with that, is that the 750GB already offers marginally higher STRs than the raptors do. but when it comes to overall application performance and responsiveness, high STRs alone usually arent what matter, more the opposite actually.

For servers, sustained transfer rate (STR) is only a piece of the puzzle. Access time and I/O Operations per second (IOPs) mean more than STR.

Obviously, the Raptor has the advantage in access time over the 7200 RPM drives due to the higher (10K RPM) rotational speed. (The Raptor's advantage goes up even more if the 7200 RPM drives are in a RAID, because the access to data time goes up when you use RAID).

As far as IOPs, the Raptors destroy every other SATA drive on the market. Take a look on that StorageReview page at the "IOMeter File Server - 2 IOs" and "IOMeter File Server - 4 IOs" benchmarks. These two tests represent a lightly and medium-loaded server, respectively. As you will see, there is a large group of SCSI and SAS drives on top, then all the WD Raptors, then all the cannon fodder.

By the way, we're talking about the Raptor(s) here to be used for the data drive(s) on the server - i.e. the drive where the server's content is going to be served from (be that web pages, files, or whatever). The system drive of the server matters a lot less, and in fact is generally chosen to be in a RAID 1 configuration for redundancy.

What's the network speed and architecture?
Is the network card a slow old PCI bus or one of the newer PCI-e cards?
No point in having a fast HD if the network is going to bottle-neck.

My internet speed is 15mb download and 2mb upload.
This server is only accessable from WAN.

I didn't buy a network card yet and I didn't know if I need a "faster" network card. And I didn't know a PCI network card would be an issue here.

This server is mainly for a web server to browse through.

And another thing: Is Gigabit ethernet card is the same as PCIe network card?

How about this: Intel® PRO/1000 PT Desktop Adapter?

Good Choice.
BTW: what board are you using?

2 Mb upload is low enough that virtually any network card will work just fine. I was under the impression that this was an internal server or was going to be doing some work on a LAN, not a WAN.

No. PCIe is the slot interface. It's newer and faster than PCI. Gigabit refers to the network (Ethernet) speed. Gigabit = 1000 Mb/sec, vs. most other network cards which are Fast Ethernet = 100 Mb/sec.

Excellent network card, but a bit overkill for this application.

Given that this server is going to serve web pages and not files, and will be doing so over a 2Mb WAN connection, the server's requirements are lightened considerably. You probably will serve the web pages just fine using a mid-range network card, and don't necessarily need a Raptor or other enterprise level hard drive. How much enterprise-level equipment you want to use is somewhat dependent on how many hits you get to the site and how mission critical the reliability of the server is, so you may have to use your best judgement there.