Newegg.com - AMD Phenom II X3 710 2.6GHz 3 x 512KB L2 Cache 6MB L3 Cache Socket AM3 95W Triple-Core Processor - Processors - Desktops Phenom II x3 710 $99
or
Newegg.com - AMD Phenom II X3 720 2.8GHz 3 x 512KB L2 Cache 6MB L3 Cache Socket AM3 95W Triple-Core Black Processor - Processors - Desktops Phenom II x3 720 $119
Newegg.com - POWERCOLOR AX4870 512MD5 Radeon HD 4870 512MB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards 4870 512mb
$127 - $10mir
or
Newegg.com - HIS H487FN1GP Radeon HD 4870 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards 4870 1GB $150 - $20mir
or
Newegg.com - HIS H485FN1GP Radeon HD 4850 1GB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards 4850 1GB $117 - $10mir
A cpu and gpu combination from those would probably work pretty well. Have to stretch budget a little bit but after rebates you could get a 4870 1GB and Phenom II x3 710 for $230. But Personally I would get the x3 720 for the unlocked multiplier and a 1gb card since GTA IV is not only cpu intensive but also gpu memory intensive. Also maybe check around for combo deals and check slickdeals.net for deals, theres been a decent amount of 4870 and Nvidia gtx 260 deals going on lately.
EDIT: found this on slickdeals
MSI R4870-T2D1G Radeon HD 4870 1GB GDDR5 PCI Express x16 (2.0v) Video Card Retail at ZipZoomfly 4870 1gb $141 - $20mir. So this and a x3 720 would come out to $241 after mir.
