Seeing as I'm pretty fed up with my system choking on TF2 when it really shouldn't... UPGRADE TIME!
This will probably result in some major juggling of computers (or components) around the family, so in hopes of your good advice I think I will post the whole deal. (I know this is a post of Teatime-esque proportions... Sorry.)
Here's what we have:
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#1: My dad's work box.
IBM Netvista M41 (or somesuch) Slim; P3 600, a Gig of RAM, <100G hard drive, no notable graphics card.
#2: My box at my parents'
FujitsuSiemens cheap-ass, Core2 (Duo I think), 2G ram, 500G Disk, OEM GF9x00 128M or somesuch
#3: My box at uni
MEDION box, Pentium D 3GHz, 3G RAM, 250G internal disk, 1,75T external disks, GF8800 GT 512M
#5 to 7:
A backup box (Linux, Raid1 over 2 1T disks) and 2 Laptops of no concern here.
A little History:
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A friend of mine at some point grabbed a bunch of #1-style boxes from eBay, for 40 bucks I just couldn't say no and replaced my dad's antique work compy with that. That one is now becoming annoyingly slow for my dad who in turn becomes annoying towards me.
When (I think) 2 years ago my then-box-at-my-parents' broke (A7V133 + Athlon 1.6GHz) we bought #2. This box has a VIA chipset and SEVERE issues (Memtest OK, Graphics card burn-in OK, Prime95 OK). It just freezes up in the middle of anything when the chipset feels like it.
#3 was bought when I started studying some 5+ years ago and is the oldest, but most reliable machine here.
The requirements:
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A) I want a box that does TF2 without too much complaint. I'm talking an absolute minimum of 20fps here.
B) My uni box has to run Hack OSX.
C) My dad needs a more recent (snappier) work box.
D) The freezing of #2 has to stop.
E) I would like to occasionally play a game on #2.
The Plan:
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Since I am pretty sure my TF2 issues stem from my mainboard, I'll just get a new one. Since I'm staying in the LGA775 class I can swap components quite freely I hope.
The requirements for OSX in almost-native mode are: ICH10, ALC889, and SSE3 capability. This works out to these Boards: MSI P43-C51, GA-EP45-UD3R (being phased out)
#2 (sans board) goes into the Athlon case with the new board, swap the graphics cards, and will become my uni machine. #3 replaces #2. This should solve A,B,D. I think I can live with E only at 9fps.

Into the case of #2 goes an ION board for my dad's new work machine, which I think is the most cost-effective way of solving C. Alternatively he'll go and buy a new laptop and use that as work machine.
Your task, should you accept it
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Well, fleshing it out into writing and juggling the options around already helped my internal decision process. If you want, you can call me a mad computer scientist (see what i did there?), amuse yourself on the fact that I've used #1 and #2 throughout this post regardless of their connotations, or you might do something productive and have a better idea, which you would then tell me.

Thanks for your attention.
tl;dr: suggest me a cheap board that runs OSX.