Here's a list of what I put in the system I built last year.

CPU AMD Phenom II x$ 955 Deneb Cores
MB  MSI 790FX-GD70 AM3 motherboard
Video  XFX ATI Radeon HD 4870x2 PCIE 2GB VRAM
Sound  (recently updated) AzunTech Cinema Explosion 7.1 (beats the heck out of my old SB X-Fi Fatality card)
RAM  8.0GB Dual-Channel DDR3 @ 666MHz  9-9-9-24
Hard Drives  500GB Hitachi HITACHI HDS7250SASUN500G 0632KKU9MF ATA Device (IDE)    101 °F
300GB Maxtor Maxtor 6B300S0    SATA
1500GB Seagate ST31500341AS    SATA
1500GB Seagate ST31500341AS   SATA
500GB Seagate ST3500630AS   SATA
500GB Seagate ST3500630AS   SATA
500GB Seagate ST350063 0AS   SATA
500GB  Hitachi (Removable ESATA external)

Optical Drive is an LG Blue Ray.DVD-RW SATA drive.

I put the whole thing into an Antec 1200 case (Full Tower).  I used a Kingwin 1000W modular power supply.
Add in a Logitech Desktop Wave Keyboard and mouse, and a decent LG W2361V 23" monitor.

Like Pauljs said, the difference is night  and day.  I went from a dual core Phenom with 4GB to this and the extra 4GB of RAM really helps.  You need to upgrade to a 64Bit OS to take advantage of the full 8GB though.  I put Windows 7 Ultimate 64-Bit on it.  Which is a big improvement over older Windows versions including XP.

This cost about $900 to put together last year.  You could probably build something like this much more inexpensively now.  And it's still a good mid line system even though technically it was obsolete a week after I built it. LOL (I should have held off for the ATI 5800 series. LMAO.)

I built this with Carrara in mind.  I haven;t had a memory dump, crash, or bog-down since I put C8 Pro 64Bit on it.

It benchmarks a bit above a comparable Intel Core i5 equipped system and comes very close to their i7 chips.

I HIGHLY recommend going with a 64Bit windows and filling up your RAM as much as possible.  That 4GB limit on 32Bit OS's is a MAJOR drawback when working with Carrara (or any other decent 3D program for that matter).  And get a full tower case so you don't run out of hard drive space, as well as spaces for more hard drives.  Some Carrara files can be pretty large and when you figure in very high resolution texture maps, and all the other stuff like Poser content, that you will accumulate over the years, you can run out of drive space quickly.  So you need to be able to expand accordingly.  My system is huge on storage and I have at least 2.5TB just in 3D content and software.  Add in other diversions like games, movie collections, music etc and it's gone before you know it.

You will note that I chose to change over to an Aren Azuntech Cinema HD Explosion 7.1 sound card.  I did have a Creative Labs Soundblaster X-Fi Fatality Pro series card, but it started going a bit wonky.  I finally got sick of Creative labs total lack of regard for their customers, bad customer service, and overpriced hardware.  I did some checking around and the card I got was highly recommended.  It sounds a lot better than the CL card, and it's about half  the price.

I hope this gives you some ideas.

Good luck.

Rich.