I think the biggest problem is the poor gfx card inside the cheaper Macbooks (Intel GMA X3100) and in the MacMini (Intel GMA 950), also the harddisks which are only small 2,5" versions, mostly with 5400 rpm. But with your OSx86 systems you won't come along, they're too slow to have fun with Final Cut. I have two nForce 4/Socket 939 systems equipped with an overclocked Opteron 185 Dual-Core (2,81 and 2,97 GHz, varies), this is great, but you can pay up to 200 Eur for an Opteron. If you have $1500 you have two choices:
First: Notebook
There are a lot of suitable notebooks with working components and a high resolution display. The speed with an Intel Core 2 Duo or similar is no problem, and a mobile nVidia graphics card will be ok I think. But I won't recommend 17" notebooks, they are much too large to be portable. But I also found 15,4" notebooks with 1680x1050 resolution. Problems: The limited internal harddisk space (although there are notebooks with two harddisks inside), 7200 rpm harddisks are expensive, and the very low batteries, today's notebooks will only last 2-2,5 hours.
Second: Desktop system
You can buy a suitable Intel desktop configuration, and check for price and power. OSx86 already supports the Intel Core i7 cpus, but I presume they are very expensive. Maybe take a look at Server CPU's, since I have Opterons I have a feeling that they are better than consumer cpus. Apple also uses for the MacPro Xeon cpus, not consumer stuff like in the other Macs. But don't ask me about Intel stuff, I'm and AMD geek ;-) You will need a good graphics card, even the ATI Radeon HD48XX Series is supported, but they have up to 285W power consumption! I'll try a ATI Radeon HD2600XT next time, it has 50W and acceptable data. Take a look in the Wikipedia for this, at least the german one has good comparisons for gfx cards. Hard disk space is no problem on an OSx86 system, you can hook up a lot of Terabyte HDs. I can recommend the Samsung Spinpoint HD103UJ (not UI !), they are amazing fast, very quiet and aren't getting hot, always below 37°C/98°F. I have an 2006'er mainboard and could use 4,8 TB in it, try this on a real Mac! They are rather cheap, in Germany about 90 Eur incl. shipping.
Third: Firewire
If you need Firewire, check for a Firewire 800 card, even if your board already has Firewire 400. They are available as PCI (often 32/64bit-cards which are a bit longer) or as PCI-E 1x cards, mostly with 3 ports. Firewire 800 is fully compatible to Firewire 400, it has twice the speed and the cables can be much longer. It has different connectors, but there are adapters available. I have in every computer a FW800 card, it is the fastest external device, eSATA isn't hot-plug-capable in OS X.
Fourth: Audio
If you have some money left and you need a good audio interface I can recommend M-Audio devices. I have two, and they're great, and rather cheap. Take a look ath the M-Audio
ProFire 610, an Firewire 400 interface which can record up to 6 channels simultaneously, ten outputs, midi interface and S/PDIF input/ouput. It supports up to 192 kHz (DVD quality!) and has a great MacOS (and Windows)
preferences tool which allows very complex settings and routing. The most interesting thing is that M-Audio products can work with ProTools M-Powered, and a (real) ProTools system is State-of-the-Art in audio studios. For a professional sequencer ProTools M-Audio is rather cheap with only 299 USD.