E5400 (pentium XEON?) for gamingm help?

Hello, I'm in need of some advice.

I've been meaning to buy me a new PC, as to let my brother use my 'old' new computer (it's a phenom II x4 955be, with HD6770 and 8gb ram)...

Anyway, my previous ancient PC was a single core pentium so other than youtubing I consider it to have reached it's dead end (actually impressed it still works tho).

That's going to take me a while to save money and all, so, my girlfriend (which happens to go to my place a lot too, but we can't play many games together since the old pc can't handle them) presented me with the alternative of trading my old pc for hers (so we can play together at my place).

So, her PC (according to Everest Home) is something as follows (Overclocking tab):

- Processor type: Intel Pentium III Xeon
-CPUID: Pentium(R) Dual-Core CPU E5400 @ 2.70 GHz

Motherboard name is listed as Unknown, it has the Lakeport-G i945G chipset.

From DXDiag it has an integrated HD 4300/4500 series (hopefully this will give hints of it's age).

I'll provide any other info asked (not sure what info is needed >.<).

From there the plan was to add some ram (it's got only 1gb), add a low range gpu (considering either gt430 or hd 5570 or 5670)... Might slap in a psu if needed.

However, I'm not sure how the CPU part is hanging. Is it going to need some overclocking?

As far as games go, it's nothing too fancy. The idea was some games in the lines of Source engine ones (l4d2, portal 2, tf2), Dead Island, and (when she's not there, why not, a bad company 2 with my brother =p).

The main idea though would be Source Engine games.

Thanks in advance,

-Dan

edit: Title wasn't very clear after reading it...