Which Windows version should I get?

Hello there. I'm currently running Ubuntu, but I've decided I'd really like to switch over to Windows. I'm working with an older machine and I have a copy from the Retail Store.

Here are the specs I'm looking at:
hp-compaq-dc7900-small-form-factor
description: Computer
width: 32 bits
*-core
description: Motherboard
physical id: 0
*-memory
description: System memory
physical id: 0
size: 3679MiB
*-cpu
product: Intel(R) Core(TM)2 Duo CPU E8400 @ 3.00GHz
vendor: Intel Corp.
physical id: 1
bus info: cpu@0
version: 6.7.10
serial: 0001-067A-0000-0000-0000-0000
size: 2750MHz
capacity: 3GHz
width: 64 bits

description: IDE interface
product: Chipset
vendor: Intel Corp

Which version of Windows would be the best fit for this setup?

I’d say Windows 7 64 is probably your safest bet. Home Premium is fine; I guess Ultimate and Professional just come with extra features most people don't even touch anyway.

Maybe try Windows 10 first and just see how it holds up? I don't know what else you're running in that rig, so who knows how the drivers will behave...

I don't actually have a dedicated graphics card—just whatever integrated chip is built into the motherboard... the whole rig was put together at the local Best Buy--

thanks.

Windows 10 64-bit Professional

Honestly, there’s zero reason to stick with Windows 7 anymore. I mean, seriously, unless you're stuck in some corporate office where nobody wants to shell out for new licenses, why bother? And from what I've gathered, the upgrade is basically free anyway. Windows 10 isn't even hitting your system harder than 7 did, and it doesn't really have "better" features in terms of raw power.
If anything, 10 actually has the best feature set, looks way cleaner, and you can snag a key for like three bucks if you know where to look online.
But hey, that's just my two cents. I know there's still a massive crowd clinging to Windows 7, but Microsoft isn't supporting it anymore—they've put all their eggs in the Windows 10 basket.