I got a Core 2 Duo E 8400, not OC´d, 4 Gigs RAM, 9800 GTX+ running on Vista 32 bit.

ROF runs great in one on one and free flight, but it stutters bad in combat with 4+ planes, although everything´s on low in the settings.

What PC do I need ( without spending too much ) ?

Well i'm not really in position to tell you what PC you need, but...I don't know, it seems a bit strange to me that your PC stutters bad with 4+ planes...I think you should rather consider this being a technical problem of some sort, and not a lack of performance of your PC.

I play RoF on a Toshiba P300 1el laptop. CPU is an Intel Core 2 Duo T5800, GPU is a Radeon HD3650 512Mo.

In free flight, i did the test a few hours ago, i get 20 FPS when on ground, and in flight around 25 FPS.

FPS almost don't drop when there are a few planes in the sky, BUT when there are 10 + planes, it becomes unplayable. It doesn't really stutter, it feels more like some sort of lag, probably due to CPU shortage.

Cheers