A cheap upgrade to my xp pc

[CrabHead]

Ayo, me again.

So I have this Dell Optiplex 755, I'm using it as a Win XP Gaming pc. the specs are:

CPU: Intel Core 2 Duo E4500 (Socket 775)
GPU: Nvidia Geforce 210
RAM: 3GB (4GB previously but one stick doesnt work anymore)
HDD: around 200 gb of storage

I have a dual boot with Win XP and Win 10 on this computer, Win XP for gaming and Win 10 for any emergency needs like some mod doesnt work on XP but works on 10.

HOWEVER... some games don't run very well on high specs, i'm talking about games from 2000's. Knight Rider The Game used to run really well on my brother's old pc which had a pentium D and same gpu with high settings, but it runs way worse on this dell rig.

I've played some Postal 2 with the MSI Afterburner OSD on (knight rider crashes for some reason on the gaming rig now), and it looks like the GPU is bottlenecking the CPU. The cpu barely rises to 50% while the GPU is constantly working at 100% non stop. The game itself runs like 20-40 fps constantly changing on medium settings.

So the GPU is the problem here. Now here's my question. What GPU should i pick for an upgrade? My budget is around 20-25 usd but even still i want it to be below 20 usd (If possible). I've thought of getting a 550ti, but i think that's a bit too overkill for a pc like this...? but i don't know.

I've been also planning to upgrade the CPU, as core2 quads are very cheap but i need to pick the GPU first then the CPU.

Any ideas?

P.S Note, There's not a lot of room in the case in terms of length. The geforce 210 is around ~13cm long (4 inches), and there's like ~4 cm (1,5 inches) before it hits the giant cooler this pc has.