I have my Macbook Pro, Its getting on in age now but its still perfect for what I use it for which is Photoshop, Internet browsing and IM'ing.

I'm looking at the possibility of making a budget gaming build so I can play games at a decent FPS and have a windows machine incase I need to use windows (there have been times) and Incase anything goes wrong with my mac (God forbid, couldn't live without this).

Anyway, briefly the estimated specs of this build are:

2-4GB DDR2 ram
AMD Athlon 4000+ x2
500gb Hard drive
22x DVD-RW
400W PSU
ATI Radeon 3850HD 256mb

I could purchase this build for around the £180 mark and re use some old parts I have lying around.

My question is, What do you think of this build and do you think it will be powerful enough for perhaps some WoW-ing, the new Command and Conquer games such as RA3 and will it be powerful enough for games like fallout 3.

My definition of powerful would be playing games at my monitors native resolution 1440x900 @ medium-high quality and getting 60FPS.

Any thoughts would really be appreciated

I wouldn't go for AMD, it is terrible ton install osx on that.
Choose Intel, sometimes a bit cheaper, but also a bit faster.
It had much more copatibility. Check this forum for tutorials (genius bar) for installing it on an Intel board

Same advice here. Get a Gigabyte GA-G31M-ES2L with a E5200 or even an OC'd E2200 and a GC with 512MB GDDR3 if you want to play. A 9600GT would be easier to install too.

Which parts do you already have lying around?

The parts I already have Laying around are basically just the Hard Drive.
I mean, I have a complete Celeron D system sitting on my desk but it's painfully slow now and Socket 478, DDR ram - which is getting pricier and pricer to upgrade.

So the low low low end intel stuff has better performance than the AMD - bear in mind that I have my Macbook Pro for OSX and this is going to be a windows build.

If you're building for Windows-only, either machine would work fine, but the Intel-based machine (GA-G31M-ES2L + E5200) would be better.

Also, keep in mind this setup would be perfectly compatible in case you ever do want to dual boot OSX on the machine. It's easy on the G31M, OSX runs perfectly on it, and at least you have the option. With AMD, you just get the average Windows clunker, nothing more.

Would it be a bottleneck if I had a CPU like that with a (Relatively) Powerful graphics card like the 3850HD?
And if I came to upgrading this rig and putting say a 4870 or a GTX260 in there would it change OSX capability by a large degree or would it be ok?

I may look at Intel, tbh I am an intel person it just seemed like I could get a cheaper Rig from AMD.