sever texture pop is often the result of a fragmented hdd.
use a dedicated defrag tool like ultimate defrag. with this kind of tool you can move your games further towards the outside of the hdd. if you have your pagefile off the primary partition move it back on to it. also make sure its set at 1.5 times your actual ram (for you thats 3 gig) no more or less.
also make sure you install your games on the primary partition.
personally i set aside 1/4 of my total storage for my primary partition, that way i can install most of my apps on the fastest part of the hdd so minimize in game lag and texture pop.
look at your gfx panel (ntune) and select the games settings for gta 4 and make sure your set on multi sample and not super sample.
1 other tip as you only have 2 gig of ram get gamebooster, it will close some background tasks and free up some much needed ram.
1 thing i do know with gta is you dont want to have a huge draw distance on a 1gig card. you need at least 1.5 gigs to set it higher than 75 percent and i personally wouldn't recommend over 50 as it takes up ram that can be spent on other things like textures...