If you're going to spend $400, you'd almost certainly be better off buying a completely new computer. I spent less than that a year and a half ago on a computer that would run circles around that Optiplex.
I did a quick search, and it looks like that computer has a Pentium 4 processor, and one of the slower ones, meaning it's quite likely that
1) it uses DDR memory, and not DDR2
2) it probably doesn't have a PCI Express port for a new graphics card
3) it might not have SATA connectors, which would severely limit the hard drives you could install
(Bit of perspective: My computer has an Athlon X2 4000+ running at 2.1ghz. That "4000+" is marketing lingo that says that the processor compares to a Pentium 4 running at 4ghz speed. The processor in the build below is about 4/3 as "fast" as mine comparing clock speeds, and is a newer architecture that's even faster. Not to mention it's dual core, which is like two Pentium 4s working together. Yes, it's a huge difference.)
And although I've never used Maya, from a glance at its wikipedia page I can tell you that it'd greatly benefit from having a graphics card.
CPU: AMD Athlon II X2 245
- not the fastest, but incredibly better than a Pentium 4
Motherboard: MSI 770-C45 AM3 AMD 770
- think this will work, not entirely sure about the new AM3 socket + DDR3 memory.
Memory: 2gb DDR3
- it's cheap and uses DDR2
Memory: 2gb DDR2
Case: Cheap Rosewill case
PSU: Cheap Rosewill power supply
- could probably find a better one, but this one works for me
Cheap DVD Burner
All together, this comes to ~$372 (motherboard needs a graphics card, see below) or ~$250. All it should need is a hard drive and video card.
For a video card, a good budget start would probably be the XFX 9800GT
- not because it's a budget card, but because it's still a fairly powerful card, it's much less expensive than the competition, and it supports CUDA (a programming language that uses the graphics card along with the CPU for more power). That adds ~$100 to the price.
Then for hard drive space, it really depends on how much storage you need. Unless you have a massive amount of movies and music, something like this 500gb Seagate ($60)
will probably work. You can always add more later on. If you're willing to spend more, they have this 1000gb Seagate ($90)
for just a little more.
There's no OS included - I'd recommend using the Windows 7 release candidate until the actual version comes out for purchase. It's free and will last until March or June (I think) of next year.
All together with the smaller hard drive, first build option comes to about $430 (you get a faster kind of memory) and the second comes to right around $410. It's slightly more expensive than the upgrades, but it's definitely more powerful. And, you can always use the old computer for something else.