That PC is still pretty good. OK, maybe not in Crysis at 1920x1200, but I think you can still sell it for a good price.
Let's start with the obvious things.
Microsoft Windows Vista Home Premium SP1 64-bit $100
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116488
Not much choice here. You need DirectX 10 and access to lots of RAM so it has to be Vista 64-bit. Your other options are Vista Basic/Business/Ultimate but they make less sense for a gaming box.
Case: Antec 900, Antec 1200, CoolerMaster RC-690, CoolerMaster HAF 932, NZXT Tempest, etc. Personal choice here. I'll assume NZXT Tempest because it's a great deal at $90 with 6 fans. I also like that it has air filters.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811146047&Tpk=NZXT tempest
If you decide to reuse your Antec 900, that's good too.
CPU: i7 i920 $295
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115202
Since you picked an i7, now you need an x58 MB and DDR3. You want something good and have a good budget, so we'll go with 6 GB rather than 3 GB.
G.SKILL 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 $155
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231230
# Timing: 7-7-7-18
# Voltage: 1.5V – 1.6V
ASUS P6T Deluxe LGA 1366 Intel X58 $300
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131346
Add a SH-S223Q burner for $30.
http://www.newegg.com/Product/Product.aspx?Item=N82E16827151173&Tpk=SH-S223Q
So far it's about $1000 including shipping costs.
Now you have to decide what to do about PSU/GPU(s) and hard disks and CPU cooling. Total budget left for these $1000.
Some options for PSU and GPU(s):
GTX 280 SLI (two cards for now) with a 1200W PSU add the option to add a third GTX 280 later.
GTX 280 SLI (two cards, no plans to add a third) and a Corsair 1000HX
GTX 295 (and no plans to add a second, benchmarks show it's not worth it) plus an Antec TPQ-850 850W PSU or a PC Power & Cooling Silencer 750W
HD 4870 X2 + HD 4870 1GB + Corsair 1000HX
HD 4870 X2 + Silencer 750W (no plans to add more cards)
The last two options would allow you to save $50 to $100 with a cheaper X58 MB that supports Crossfire but not SLI. Gigabyte makes some of those.
About the HDD: I like the WD6401AALS. You could also consider a Velociraptor, but IMO that money is better spent on graphics cards.
Cooling: if you want to overclock get a Noctua NH-U12P 1366. If not, don't buy any aftermarket CPU cooler.