CRT monitors are the best as far as cost goes. After all, I got my 1920x1440 monitor for only $150 via retail (not Ebay). The 1920x1200 LCD goes for more than triple that (roughly $500) and it's even less resolution. The next highest is 2560x1600, but that costs double again and it's not available on a CRT (2048x1536 is the highest I've seen for CRT monitors).
The i5-2500K (about $220) is a decent mid-range processor that can supposedly be easily overclocked if needed.
The GeForce GTX 465 (about $120) has the highest benchmarks for processing power over cost - the best for the least). Since animation type stuff usually involves the video card, you might consider a more powerful card and the Radeon HD 6850 seems quite good for a price to power ratio but it costs roughly $175. Given benchmarks and your budget, the HD 6850 is likely the best option. Higher up is the GeForce GTX 480, costing a rather hefty $300. Much beyond here, the price to power ratio is pretty weak.
The CPU and video card alone cost about $400, assuming the recommended video card. Throw in a semi-decent motherboard ($100), RAM ($20 for 2 GB though I'd recommend up to 8 GB if you're doing high end stuff, of which hovers around $80), a good monitor (a 1600x1200 CRT hovers around $125), and other standard necessities (CD drive, keyboard, mouse, case, power supply, etc.), and you should be pretty close to your $800 max.
For reference, the benchmarking site I used is PassMark:
http://www.videocardbenchmark.net/high_end_gpus.html
These benchmarks are likely geared toward gaming, not animation. As to what kind of PSU you'll need, eXtreme Power Supply Calculator is pretty good - always add at least 10% to the recommendation, preferrably 25%. You might add on more as funding becomes available. This calculator is available here:
http://extreme.outervision.com/psucalculatorlite.jsp
Hopefully this will help you get started.
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