Can the Cube be current? Sure. Mine is. There are a lot of people out there that still have a Cube.
Currently, I'm working with
-OS X 10.4.8
-Powerlogix Dual 1.7GhZ 7447A
-320GB Seagate HD
-SuperDrive
-1.5GB memory
-Geforce 5200FX w/256mb onboard
-Airport Card
-Bluetooth Dongle
-(1) base fan and (1) video card fan
-Creative Xfi XMod USB audio
Dude up there takes the win with the 9800, but I am assuming that would be with a Powerlogix upgraded cube case. No way a 9800 will fit in the stock Cube case. Cost for cost, it could be reasonable to estimate it at the cost of a higher end imac, I suppose. But the upgrades can be done in small bits, making it easier to deal with. While there are no expansion ports and only one 3.5'' bay (if you take out the optical drive, fitting another HD would be possible), there are 2 FireWire ports that work great. I've got a couple of external drives that I use, and FireWire is also bootable. The upgrades themselves vary in difficulty, from quite simple to rather involved, depending on what you are doing. A new HD and installing the ATA hi-cap extender kernal extension is not difficult, but moving the VRM to fit crazy video cards in there is a big project. Swapping the processors out took some time, but if you follow the process and instructions carefully, its not bad.
I think at this point, its probably still relevant as a computer, and I fully expect to use mine with Leopard and later, and will make further upgrades if they become available. Its a fun project, but it can be pricey. I initially started this as a side project, but the Cube quickly became my main machine. It's quite reliable, provided that you keep it cool and don't push too many limits that shouldnt be pushed. There are solutions out there for making the Cube more or less "current" by today's standards, (excluding USB 1.1, that cannot be changed) you may just have to look around. Currently, the max processors available are 1.8GhZ Singles or 1.7GhZ duals, but the dual 1.7s are quite hard to find as they stopped producing them. I am hoping for something over 2GhZ, but I dont know if that is possible or even safe!
I would recommend checking out
www.cubeowner.com
A total site for anything you need to know about your cube. Super cool people, too. Enjoy your Cube!