Welcome back! Are you planning on gaming, and what resolution is your monitor? Intel are generally still favoured, not much has changed there since your last CPU purchase If I gather correctly, you will need: Operating System CPU/Mobo/RAM/GPU Possibly a PSU depending on how much you spent last time. You can recycle the X-Fi, case and DVD drive for now (and XP if you're happy to, but at least 7 is highly recommended). You won't get a vast amount for £400, but it'll be a big step up anyway.
intel is still the way to go for 3d, allthough rendering could actually be a little bit faster on AMD-CPUs due to them having more physical cores (benchmarks like POVRAY and Cinebench show this trend atleast), but not in your budget-range. Ontop of this you need as much DDR3 as possible, whereas the GPU is totally not as interesting. A HD7750 will be enough here. I'm doing lots of photoediting, SketchUp and 3dmax on my rig aswell (see signature) and I can't complain, but more RAM is recommended, if the rig is specificall ment for this task. £400 is really not that much to play with however, but it'll do. intel Core i5-3470 @ £150.18 incl VAT MSI B75MA-P45 @ £46.50 incl VAT Corsair XMS3 16GB DDR3 1600 MHz @ £93.79 incl VAT Gigabyte HD7750 OC 2GB @ £71.72 incl VAT = £362,19
You could even recycle your X1900XTX if your applications aren't GPU accelerated and you won't be playing games. Which power supply do you have specifically?
The 7750 is much, much faster than the x1900xtx. It is roughly equivalent to a top end card from about four years ago, whereas the old xtx is at the ripe old age of seven now. Windows 8 is already out, so it's probably the de facto choice now. 16GB is so cheap compared to last time you looked, under £100
Depends on your definition of cheap I suppose High end boards are around the £130-£150 mark, whereas a decent ATX board can be found for a little less with more features. That said, you could get a nice Bitfenix Prodigy and put a potent little CAD box together for your budget, as GPU horsepower isn't a major requirement.