Maximum PC

I've been playing Grand Theft Auto IV on the pc that I built, and I noticed that it's lagging, especially during map rendering. I want to upgrade, but I'm not sure what I should upgrade.
Here's what I have now:

Intel Core i5-2400 3.10Ghz
GeForce GTX 550 Ti
WD 320GB HDD
4 Gigs of ram (Two G.Skill Ripjaw DDR3)
MSI H67A-G43 B3 Motherboard

Would upgrading to an SSD help (Considering the problem is mostly rendering, I'm thinking the HDD might be slow to get game data out)? I've been wanting one for a while, but the prices on them is still up.

If you mean lagging by unresponsive, then yes, it's your storage. Upgrading to an SSD would help, if you put the game on there.

ok thanks. The prices for SSDs are still crazy, though. Would you recommend getting something like a 50 gig SSD to go along with my HDD and only install certain programs on it? And if so, should I reinstall my OS on it?

Yes, the prices of SSDs are crazy compared to the prices of HDDs. I can get a 1.5TB HDD for $80, so I do think that $450 for a 300GB SSD (which is $1.50/GB) is a bit high. You're right about getting 128GB, though. From that point down seems to be really inexpensive for some reason. There's plenty of 128GB SSDs for under $90, but jumping up to 160GB adds $100. Weird.

SSDs are a high performance part, not a high storage part. The fastest HDD can boast a sequential read time of 150/MBs with a few milliseconds of seek time. The fastest SSD boasts a sequential read time of at least 550MB/s and a seek time of less than 0.1ms. SSDs run circles around HDD in terms of performance.

You're paying for the speed the speed of secondary storage, which will address the problem you have presented.

Also there's nothing weird about "asinine" price jumps. You see this everywhere in computer parts.