mobile is i7 6500u 2.50 ghz
Just to get this out there, you have to be very careful when comparing mobile vs desktop CPU's. With desktops its easy. Pre-8xxx an i3 was two cores with hyper threading and no turbo. An i5 was four cores, no hyper threading but with turbo. i7 was 4 cores with hyper threading and turbo. The mobile world is much more difficult to figure out. There were some i7's that were 4 core, but most were two core. They one you are talking about is a 'u' CPU which is ultra low power. Its a two core with hyper threading and turbo and running at lower voltage and clock speed for power savings. To complicate things further, an i5 xxxxu is also two cores with hyper threading and turbo. In most tests there is almost no difference between an i5u and an i7u and neither one will come close to a desktop i5 performance wise. Anyway, in any kind of gaming comparison, pretty much any desktop CPU is going to out perform the 6500U .
Here is a resource I always point out in discussions like this. For any Intel CPU, google ARK then the model. In your case google "ARK i7 6500U" and you will get the full specs on that CPU. When making these comparisons I open two browser windows and google ARK CPU1 in one window and ARK cpu 2 in the other window.. Then I can look at the two side by side.
On the "new" desktop, here are some questions to ask about it:
1. Is it actually new or is it used/refurbed? Around where I live, there are places that buy old desktops from businesses where computers are often on a pre-defined replacement cycle...meaning that PC's that are still in good working order get replaced when the support agreements run out. They give them a checkup and then re-sell them. These can be an OK deal if the price is very low compared to an equivalent PC that's actually new. Sometimes there is some kind of warranty offered, sometimes not. You don't always know what you are getting
2. We have other places where they are selling PC's that are old generations but actually new and unused. I see a lot of Dells show up in this category. I assume they must have been sitting in a warehouse somewhere unsold. Also a possible good deal if they are cheap enough.
3. In pre-built, you can probably find a newer generation than ivy bridge that doesn't cost much more or any more. Example: here is a machine with an i5-7400 for $399.
http://www.microcenter.com/category/4294967292,4294848341/PC-Desktops . The same place is selling refurbed Dell desktops with i5-4570's for anywhere from $249 up to $389. With a similarly speced new 7400 only $10 more, the $389 for a 4570 refurb (at the same reailer!) would be a non-starter
4. With a lot of older machines, be it refurb or actually new, replacing the old spinning disk with even a cheap SSD is going to make it feel new. Stick something like a Samsung 860 EVO in there (about $85) and it will feel like a new machine. Other than that I would plan to not spend any money upgrading one of these. An SSD could be carried forward to a newer build someday but most other things would be too expensive or (like RAM) an older generation that would not carry forward.