Planning my first build - x86 or x64?

Hi guys, I joined the community a few days ago with a question about upgrading my graphics card. I've decided to go all in and go for a new build.
I've got a couple of questions, firstly should I go for 32 bit or 64 bit? I have access to [legal] versions of XP Pro, Vista Ultimate and Windows 7 (when it arrives) in both 32 and 64.

I understand with 32 bit I am capped at 4 GB RAM.

The PC will be used for general computing, casual gaming and engineering CAD.

Depending on my money situation I will most likely go with a AMD Phenom II X4 Quad Core 955 Black Edition overclocked to around 3.8GHz or an Intel Core i7 920 overclocked to around 4 GHz.

This brings me round to my next question - will the i7 be worth the extra money? I have always had Intel so am no too sure about AMD.

I appreciate any advice, as the title says, this will be my first complete build, in the past I have just upgraded.

1) Get the 64 bit of Win7! There is no reason now not to get a 64 bit system. It will allow you to see & use all your RAM. The support is much better since the days of XP 64 bit.

2) If you are going to be doing a lot of CAD work, I would recommend the Core i7 920. It os the best CPU for those types of applications and worth the price difference. In other words, I would recommend buying the Core i7 920.

I would go with bundle 3, since it is the cheapest and provides good components. If you want to overclock a lot, bundle 1 has the best MOBO & HSF for overclocking.

Sorry, another question, but I just noticed you have a VelociRaptor - have you seen a big improvement with it?

I'm deciding between a 1 TB 7200RPM or the 300GB 10000RPM VelociRaptor.

I have noticed a difference but there are better options out now for less money and similar performance. I would recommend anyone of these over the VRaptor... Western Digital 640GB Black or Samsung Spinpoint F3 500GB hard drives. Both are almost equal in speed for a 1/4 the price. If you want to pay for the VRaptors price, I say look at a SSD for much faster performance.

I appreciate the help. I'm just looking round to see if I can get the bundle you recommended any cheaper.