Before buying new kit, make sure that the problem isn't your cable or the VGA-in on the monitor. If you always connect other machines to it by DVI or HDMI then you'd not notice a deficiency in the analogue inputs normally. While the ghosting you describe could be caused by the laptop, it could also be due to the monitor's input or inadequate screening (or other fault) in the cable.
USB based graphics adaptors do not seem too expensive (though obviously more so than internal graphics cards which can be much cheaper due to economies of market scale), this one for instance which was the first to come up in a quick search. I would go with an adaptor that can drive either DVI or VGA (like the one I linked to above) for flexibility (DVI for quality, VGA for when DVI is not available).
I would search out real-world reviews of any such devices before buying. One concern I would have is the operating speed of USB (even V2). A 1920*1080 24-bit raw screen frame is just under 6Mbyte (1920*1080*3/1024/1024), and your slightly smaller resolution is still a little over 5Mbytes per raw frame. Assuming the ~25-to-30Mbyte/sec I see a the maximum inward throughput of USB connected hard-drives is due mainly to the USB bus limitations (which I believe it is) you would only get around 5 frames per second at best down the USB port at that rate. The device driver for the adaptor may compress the data before sending it over the USB connection (and the hardware decompress it before sending the image to the screen) which would greatly improve update rates but at the expense of CPU time and update latency (frame lag). If lossless compression is used I would not expect enough compression to be achieved in order for full-screen movies or games to be practical over such a device and if lossy compression is used to get update rates high enough quality will obviously suffer. Even if I'm right about this limitation though, such a device should be perfectly usable for less graphically intensive work such as office work, web browsing, programming, small non-full-screen video, and so on - as for those tasks a lot less information is being passed to a graphics card at any given time.
ps. You should add to your question brief details of what you generally use the laptop for - this will help people give more relevant answers.