If your PC was built with running Xbox 360 ports in mind, it will still continue to be able to play 360 games for the duration of that consoles lifespan. Budget quad core, 4GB DDR2, 9800 GFX card, sorted, as long as your restricting yourself to playing 360 ports.
In this age of console gaming, constantly needing to upgrade is a bit of a lie TBH. Those PC specs would play 360 ports and they would look slightly better, but, you have the OPTION, if you have more cash for cutting edge hardware, to push the sliders as far as you like. And no, the difference is not slight. Play with max settings for 5 minutes, then set them down the medium-low again, your eyes bleed. Its like the difference between TV and HDTV, you couldn't care for HD, until you'd see it in all its glory.
Back in the day, yeah, PC devs tended to develop largely for higher end hardware, so you were in for a upgrade every 2 years. It just so happened that it was during that period that we saw the biggest leaps in games, in graphics, AI and game mechanics. Devs and their games have kinda hit a hardware wall now, bring on the next gen on consoles!
This post comes off as PC fanboi, but I have a 360 as well and its a fun, easy to use platform. I have no hate for consoles, except the Wii.... I just wish they'd push out the next gen of consoles at a faster pace so we can see the bar raised again in gaming.