Since you said your old Mac isn't "as nimble as she once was", you obviously noticed that. If you want a vastly faster iMac, you can get one because technology has progressed a lot. In fact you can get an iMac that's faster than a quad-core Mac Pro.
If instead you're looking for superficial styling changes (like "racing stripes"), Apple doesn't do that. E.g, the 2014 MacBook Air looks just like the one from 2008.
So your first step is decide what you want -- are your priorities a faster iMac with more capability, more storage, faster disk, better I/O expansion, vastly better GPU -- one far more "nimble" and pleasurable to use? If so you can get one. OTOH if your priorities are to get something that looks different for the sake of frivolous, cosmetic change, you can probably find a decal kit with racing stripes or lightning bolts for your present iMac.