You'd likely be blown away with a shiny new computer - but then you have to make sure you find one that will work flawlessly with your aging PCI HDSP card. If you find a new PC that won't have a performance penalty over PCI, I doubt a new RME Interface will get you a substantial improvement over the MF you have now.
You could always track down a HDSPe PCIe card and continue using your trusty MF with ANY modern desktop PC with a PCIe slot. I still run a few MF's (one over PCI) and a Digiface along with newer RME Gear. I love all of it!
Of course the newer FF802 and UCX will have a bit better converters (and faster converters), will have a few mic Pre's, and TM-FX is killer bar-none (don't overlook TM-FX - it's a BEAST!).
Would the $$$ investment in one of these newer units outweigh keeping your MF and investing in 1 or 2 nice pieces of analog gear, descent monitors, or a descent mic? That's a question only you can really answer based off your particular workflow...
The RME USB interfaces do recommend more recent Intel Chipsets for best USB performance. What is the state of your current PC? And what outboard/monitors/mics/rooms do you have now? What's your workflow? That's really the path to a meaningful answer to this question
MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8