Haha, to be perfectly honest, I think Xeon CPUs are a big, big waste of money. I'm surprised you swapped your i7 930 for an X5560 though, considering they are essentially the same CPU inside, but the Xeon just has extra server features, such as support for ECC RAM...etc. which are of no use to you anyways.
If I were in your situation, I would have bought a dual CPU board and chucked the two X5560s on and see how they perform, that would actually be jizz-worthy, but oh well, you can't now, so oh well

To be honest, I don't think the E5504 is THAT bad, like it's the same Nehalem microarchitecture as the rest of the i7s and Xeons in that range, it's just a cut down, low GHz, low cache version, so you'll get extra zip with an i7 920/930 (which is the same CPU, except the 920 multiplier is set one step lower) but it won't be as much extra zip as you're thinking

I'll assure you that.
But anyway, I think you're a little unlucky to be stuck on the LGA1366 platform, as much as I like it (things such as triple channel RAM is nice), I think it's really a server platform and hence it really limits your options in terms of CPUs. The LGA1155 platform is probably the better one to be on, but even now, if you get an i7 920 or 930, you'd still be 3 generations behind (4 generations when Haswell comes out next year) - so really, you'll be running a CPU that's 4 years old.
I reckon, if I were in your position, I'd just spend around $300 to get an i7 3820, considering it's probably more economical to stick with the LGA1366 platform for now.
You see, I'm on LGA1155 (Sandy Bridge at the moment) - so upgrade paths are a lot simpler.

To be honest, get an SSD, you can get a 120GB SSD for $99 - it'll be a good $99
