Depending on the money you can spend on a new PC (and on your location).
Here in Germany I spent 425€ on a pretty decent machine that can run UT3 with full details with a nice performance and that can run Bulletstorm with beautiful graphics!

thats a lot of money for a just 'decent' pc :)

but as always it depends on how many parts you can use from you old rig, f.e. case, dvd drive, harddisk

In my eyes it's a "decent" PC, not high end but can run today's games with pretty decent/high quality. My only problem was that I was so lucky to have a bad start on this rig (my GPU was almost defect from the beginning and had a tendency to crash with nice stripes in the colours that were the most visible on the screen - in UT3 (before colour ini tweak) it was GREY! So don't tell me UT3 has colours lol. Then one of my two RAMs died and I just had 2GB. Never heard that Corsair RAM died without any reason and wasn't dead from the beginning - had this happen though. From one day to another it died... )

Out of the top of my head, here's my rig to give you a "starting spot":
AMD Athlon II X4 3GHz with Katana cooling (never gets even close to the "borderline of death"), 4GB RAM DDR3 with dual channel support (standard these days, by Corsair), AMD/ATI Radeon HD 5770 (quiet as hell! and I had bad luck with it), Be Quiet! power supply unit 530W (again quiet as hell + should be reliable), msi mainboard (forgot which one and too lazy to look it up) and a 500GB HDD (honestly, I don't need more. The max I will use is 350 anyway with all my old games - sadly I won't be able to run Destruction Derby 2 anymore... have to see if I can still run Duke Nukem 3D - hopefully! ).

The only things I could reuse from my "stone-age rig" were:
My ancient DVD drive (lol), my laptop mouse (lol), my old keyboard that has a tendency to not register what I've just typed on some occasions (still rare though so no need to buy a new lol), my nice LCD monitor (not 3D (pff UT in 3D, don't need that) and no full HD nor HD, honestly who needs that shyte anyway!? Just a fancy extra ) and my fancy speakers and subwhoofer.

Apart from these parts and my FANCY case this computer is comepletely new and has a two year guarantee for all parts (and more for some others, like my RAM - LIFETIME (which is 10 years here in Germany lol) ). Also, I didn't overclock it because of the guarantee. Otherwise I would lose it.

Yaz just have to remember that in Auzzie we get ripped off for most things retail by about 40% compared to other countries. eg.. in steam/itunes we pay 40% more than america for exactly the same digital download....(the Auzzie dollar is above parity against US yet an album costs $16.99 here and the US pay $9.99)... :/
$900 here will get ya a AMD x4, mobo, 4gig of ram, 1TB hdd, 560 gtx, decent case n decent PSU. Personally I wouldn't go any less than a quad core these days. Quads will last a while as well!

the 590 from nvidia beats it but not really on performances...lol: its more reasonably sized but still huge, it eats a lil less power but still eating like an ogster, it works less hot because they lowered the GF110 a lot (it s in fact like 2x580GTX lowered) but it still a toaster and it cast less noise but still noisy like a hairdryer....and to finish it costs round 650Euros...from what i heared watercooling is required  on the 6990 because the noise is so loud that it destroy users gaming pleasure

thoses bigpu cards are the nightmare of air cooling and u ll not see often manufacters trying them on special aircooled versions...lol...

Nvidia 570 & 580, ATI 6950 & 6970 are what need a classical gamer (easy to SLI or Crossfire em if u want more l8r) and here u ll find super aircooled, OCed from stock and even amiliored(better components) models for decent prices from several manufacters

... except if u use a big big screen or if u plane to bench a lot it s quite a waste of money to buy such brand new superpowered GFX's....it always hurt to buy a 600$ material and to see it for 200$ on EBay 6 month l8r...i think all what i ve read from "reputed reviewers" goes in that way

on the mobo choose , if u go on the AMD side be sure to buy a good AM3+ model with at least 2 pcie slots, with sata 6gb/s and USB3...they are often able to run Nvidia SLI or ATI crossfire...the mobo is the base and with the AMD mobo/cpu retrocompatibility politic u can keep it for years and improove it by a new cpu or a new GFX

 i hope this makes sence and can help a little